feat:light/dark mode switch (#138)

Summary

- Adds browser theme detection on first visit using
prefers-color-scheme media query
- Renames localStorage key from dark-mode to
next-ai-draw-io-dark-mode for consistency with other keys
- Uses STORAGE_DIAGRAM_XML_KEY constant instead of hardcoded
string in diagram-context.tsx

Changes

app/page.tsx:
- On first visit (no saved preference), detect browser's color
scheme preference
- Update localStorage key to follow project naming convention
(next-ai-draw-io-*)

contexts/diagram-context.tsx:
- Import STORAGE_DIAGRAM_XML_KEY from chat-panel.tsx
- Replace hardcoded "next-ai-draw-io-diagram-xml" with the
constant

contexts/diagram-context.tsx

@@ -3,6 +3,7 @@
 import type React from "react"
 import { createContext, useContext, useRef, useState } from "react"
 import type { DrawIoEmbedRef } from "react-drawio"
+import { STORAGE_DIAGRAM_XML_KEY } from "@/components/chat-panel"
 import type { ExportFormat } from "@/components/save-dialog"
 import { extractDiagramXML, validateMxCellStructure } from "../lib/utils"
 
@@ -24,6 +25,7 @@ interface DiagramContextType {
     ) => void
     isDrawioReady: boolean
     onDrawioLoad: () => void
+    resetDrawioReady: () => void
 }
 
 const DiagramContext = createContext<DiagramContextType | undefined>(undefined)
@@ -45,9 +47,16 @@ export function DiagramProvider({ children }: { children: React.ReactNode }) {
         // Only set ready state once to prevent infinite loops
         if (hasCalledOnLoadRef.current) return
         hasCalledOnLoadRef.current = true
-        console.log("[DiagramContext] DrawIO loaded, setting ready state")
+        // console.log("[DiagramContext] DrawIO loaded, setting ready state")
         setIsDrawioReady(true)
     }
+
+    const resetDrawioReady = () => {
+        // console.log("[DiagramContext] Resetting DrawIO ready state")
+        hasCalledOnLoadRef.current = false
+        setIsDrawioReady(false)
+    }
+
     // Track if we're expecting an export for file save (stores raw export data)
     const saveResolverRef = useRef<{
         resolver: ((data: string) => void) | null
@@ -171,6 +180,9 @@ export function DiagramProvider({ children }: { children: React.ReactNode }) {
                     fileContent = xmlContent
                     mimeType = "application/xml"
                     extension = ".drawio"
+
+                    // Save to localStorage when user manually saves
+                    localStorage.setItem(STORAGE_DIAGRAM_XML_KEY, xmlContent)
                 } else if (format === "png") {
                     // PNG data comes as base64 data URL
                     fileContent = exportData
@@ -251,6 +263,7 @@ export function DiagramProvider({ children }: { children: React.ReactNode }) {
                 saveDiagramToFile,
                 isDrawioReady,
                 onDrawioLoad,
+                resetDrawioReady,
             }}
         >
             {children}