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/ai-config.ts

@@ -0,0 +1,26 @@
+import { STORAGE_KEYS } from "./storage"
+
+/**
+ * Get AI configuration from localStorage.
+ * Returns API keys and settings for custom AI providers.
+ * Used to override server defaults when user provides their own API key.
+ */
+export function getAIConfig() {
+    if (typeof window === "undefined") {
+        return {
+            accessCode: "",
+            aiProvider: "",
+            aiBaseUrl: "",
+            aiApiKey: "",
+            aiModel: "",
+        }
+    }
+
+    return {
+        accessCode: localStorage.getItem(STORAGE_KEYS.accessCode) || "",
+        aiProvider: localStorage.getItem(STORAGE_KEYS.aiProvider) || "",
+        aiBaseUrl: localStorage.getItem(STORAGE_KEYS.aiBaseUrl) || "",
+        aiApiKey: localStorage.getItem(STORAGE_KEYS.aiApiKey) || "",
+        aiModel: localStorage.getItem(STORAGE_KEYS.aiModel) || "",
+    }
+}