refactor: eliminate code duplication (DRY principle) (#211)

## Problem Solved
Previous refactoring added 105 lines (1476→1581) by extracting code into separate files without eliminating duplication. This refactor focuses on reducing code size through deduplication while maintaining file separation for maintainability.

## Summary
- Reduced total lines from 1581 to 1519 (-62 lines, 3.9% reduction)
- Eliminated duplicate patterns using generic helpers and factory functions
- Maintained file structure for maintainability
- Zero functional changes - same behavior

### Phase 1: DRY use-quota-manager.tsx
- Created parseStorageCount() helper (eliminates 6x localStorage read duplication)
- Created createQuotaChecker() factory (consolidates 3 check function bodies)
- Created createQuotaIncrementer() factory (consolidates 3 increment function bodies)
- Result: 242→247 lines (+5 lines, but fully DRY with eliminated duplication)

### Phase 2: DRY chat-panel.tsx (1176→1109 lines, -67 lines)

#### 2.1: Extract checkAllQuotaLimits helper
- Replaced 3 occurrences of 18-line quota check blocks
- Saved 36 lines

#### 2.2: Extract sendChatMessage helper
- Replaced 3 occurrences of 21-line sendMessage+headers blocks
- Saved 42 lines

#### 2.3: Extract processFilesAndAppendContent helper
- Replaced 2 occurrences of file processing loops
- Handles PDF, text, and image files uniformly
- Async helper with optional image parts parameter

lib/use-quota-manager.tsx

@@ -0,0 +1,247 @@
+"use client"
+
+import { useCallback, useMemo } from "react"
+import { toast } from "sonner"
+import { QuotaLimitToast } from "@/components/quota-limit-toast"
+import { STORAGE_KEYS } from "@/lib/storage"
+
+export interface QuotaConfig {
+    dailyRequestLimit: number
+    dailyTokenLimit: number
+    tpmLimit: number
+}
+
+export interface QuotaCheckResult {
+    allowed: boolean
+    remaining: number
+    used: number
+}
+
+/**
+ * Hook for managing request/token quotas and rate limiting.
+ * Handles three types of limits:
+ * - Daily request limit
+ * - Daily token limit
+ * - Tokens per minute (TPM) rate limit
+ *
+ * Users with their own API key bypass all limits.
+ */
+export function useQuotaManager(config: QuotaConfig): {
+    hasOwnApiKey: () => boolean
+    checkDailyLimit: () => QuotaCheckResult
+    checkTokenLimit: () => QuotaCheckResult
+    checkTPMLimit: () => QuotaCheckResult
+    incrementRequestCount: () => void
+    incrementTokenCount: (tokens: number) => void
+    incrementTPMCount: (tokens: number) => void
+    showQuotaLimitToast: () => void
+    showTokenLimitToast: (used: number) => void
+    showTPMLimitToast: () => void
+} {
+    const { dailyRequestLimit, dailyTokenLimit, tpmLimit } = config
+
+    // Check if user has their own API key configured (bypass limits)
+    const hasOwnApiKey = useCallback((): boolean => {
+        const provider = localStorage.getItem(STORAGE_KEYS.aiProvider)
+        const apiKey = localStorage.getItem(STORAGE_KEYS.aiApiKey)
+        return !!(provider && apiKey)
+    }, [])
+
+    // Generic helper: Parse count from localStorage with NaN guard
+    const parseStorageCount = (key: string): number => {
+        const count = parseInt(localStorage.getItem(key) || "0", 10)
+        return Number.isNaN(count) ? 0 : count
+    }
+
+    // Generic helper: Create quota checker factory
+    const createQuotaChecker = useCallback(
+        (
+            getTimeKey: () => string,
+            timeStorageKey: string,
+            countStorageKey: string,
+            limit: number,
+        ) => {
+            return (): QuotaCheckResult => {
+                if (hasOwnApiKey())
+                    return { allowed: true, remaining: -1, used: 0 }
+                if (limit <= 0) return { allowed: true, remaining: -1, used: 0 }
+
+                const currentTime = getTimeKey()
+                const storedTime = localStorage.getItem(timeStorageKey)
+                let count = parseStorageCount(countStorageKey)
+
+                if (storedTime !== currentTime) {
+                    count = 0
+                    localStorage.setItem(timeStorageKey, currentTime)
+                    localStorage.setItem(countStorageKey, "0")
+                }
+
+                return {
+                    allowed: count < limit,
+                    remaining: limit - count,
+                    used: count,
+                }
+            }
+        },
+        [hasOwnApiKey],
+    )
+
+    // Generic helper: Create quota incrementer factory
+    const createQuotaIncrementer = useCallback(
+        (
+            getTimeKey: () => string,
+            timeStorageKey: string,
+            countStorageKey: string,
+            validateInput: boolean = false,
+        ) => {
+            return (tokens: number = 1): void => {
+                if (validateInput && (!Number.isFinite(tokens) || tokens <= 0))
+                    return
+
+                const currentTime = getTimeKey()
+                const storedTime = localStorage.getItem(timeStorageKey)
+                let count = parseStorageCount(countStorageKey)
+
+                if (storedTime !== currentTime) {
+                    count = 0
+                    localStorage.setItem(timeStorageKey, currentTime)
+                }
+
+                localStorage.setItem(countStorageKey, String(count + tokens))
+            }
+        },
+        [],
+    )
+
+    // Check daily request limit
+    const checkDailyLimit = useMemo(
+        () =>
+            createQuotaChecker(
+                () => new Date().toDateString(),
+                STORAGE_KEYS.requestDate,
+                STORAGE_KEYS.requestCount,
+                dailyRequestLimit,
+            ),
+        [createQuotaChecker, dailyRequestLimit],
+    )
+
+    // Increment request count
+    const incrementRequestCount = useMemo(
+        () =>
+            createQuotaIncrementer(
+                () => new Date().toDateString(),
+                STORAGE_KEYS.requestDate,
+                STORAGE_KEYS.requestCount,
+                false,
+            ),
+        [createQuotaIncrementer],
+    )
+
+    // Show quota limit toast (request-based)
+    const showQuotaLimitToast = useCallback(() => {
+        toast.custom(
+            (t) => (
+                <QuotaLimitToast
+                    used={dailyRequestLimit}
+                    limit={dailyRequestLimit}
+                    onDismiss={() => toast.dismiss(t)}
+                />
+            ),
+            { duration: 15000 },
+        )
+    }, [dailyRequestLimit])
+
+    // Check daily token limit
+    const checkTokenLimit = useMemo(
+        () =>
+            createQuotaChecker(
+                () => new Date().toDateString(),
+                STORAGE_KEYS.tokenDate,
+                STORAGE_KEYS.tokenCount,
+                dailyTokenLimit,
+            ),
+        [createQuotaChecker, dailyTokenLimit],
+    )
+
+    // Increment token count
+    const incrementTokenCount = useMemo(
+        () =>
+            createQuotaIncrementer(
+                () => new Date().toDateString(),
+                STORAGE_KEYS.tokenDate,
+                STORAGE_KEYS.tokenCount,
+                true, // Validate input tokens
+            ),
+        [createQuotaIncrementer],
+    )
+
+    // Show token limit toast
+    const showTokenLimitToast = useCallback(
+        (used: number) => {
+            toast.custom(
+                (t) => (
+                    <QuotaLimitToast
+                        type="token"
+                        used={used}
+                        limit={dailyTokenLimit}
+                        onDismiss={() => toast.dismiss(t)}
+                    />
+                ),
+                { duration: 15000 },
+            )
+        },
+        [dailyTokenLimit],
+    )
+
+    // Check TPM (tokens per minute) limit
+    const checkTPMLimit = useMemo(
+        () =>
+            createQuotaChecker(
+                () => Math.floor(Date.now() / 60000).toString(),
+                STORAGE_KEYS.tpmMinute,
+                STORAGE_KEYS.tpmCount,
+                tpmLimit,
+            ),
+        [createQuotaChecker, tpmLimit],
+    )
+
+    // Increment TPM count
+    const incrementTPMCount = useMemo(
+        () =>
+            createQuotaIncrementer(
+                () => Math.floor(Date.now() / 60000).toString(),
+                STORAGE_KEYS.tpmMinute,
+                STORAGE_KEYS.tpmCount,
+                true, // Validate input tokens
+            ),
+        [createQuotaIncrementer],
+    )
+
+    // Show TPM limit toast
+    const showTPMLimitToast = useCallback(() => {
+        const limitDisplay =
+            tpmLimit >= 1000 ? `${tpmLimit / 1000}k` : String(tpmLimit)
+        toast.error(
+            `Rate limit reached (${limitDisplay} tokens/min). Please wait 60 seconds before sending another request.`,
+            { duration: 8000 },
+        )
+    }, [tpmLimit])
+
+    return {
+        // Check functions
+        hasOwnApiKey,
+        checkDailyLimit,
+        checkTokenLimit,
+        checkTPMLimit,
+
+        // Increment functions
+        incrementRequestCount,
+        incrementTokenCount,
+        incrementTPMCount,
+
+        // Toast functions
+        showQuotaLimitToast,
+        showTokenLimitToast,
+        showTPMLimitToast,
+    }
+}