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

components/chat-message-display.tsx

@@ -135,6 +135,7 @@ export function ChatMessageDisplay({
     const copyMessageToClipboard = async (messageId: string, text: string) => {
         try {
             await navigator.clipboard.writeText(text)
+
             setCopiedMessageId(messageId)
             setTimeout(() => setCopiedMessageId(null), 2000)
         } catch (err) {