feat: chat stop button, image paste, vision AI, search fixes

- Add stop button to all chat interfaces (floating, contextual, full-page)
- Add conversation sliding window (50 messages) to prevent context overflow
- Add chat timeout warning (30s toast)
- Force response language in chat system prompt (mandatory per-locale)
- Add image paste from clipboard in all note editors (card, list, input)
- Fix upload API to infer extension from MIME type for clipboard images
- Add image description support in note AI chat (base64 vision)
- Fix search regex crash on special characters (escape user input)
- Fix search case-insensitivity on PostgreSQL (mode: 'insensitive')
- Add try/catch around semantic search in chat route (prevent blocking)
- Add new chat button to floating AI assistant
- Fix empty thinking bubbles for reasoning models (filter non-text parts)
- Remove duplicate AI assistant toggle from note editor header
- Improve link metadata scraping (timeout, content-type check, relative URLs)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

memento-note/app/api/upload/route.ts

@@ -33,9 +33,16 @@ export async function POST(request: NextRequest) {
     }
 
     const buffer = Buffer.from(await file.arrayBuffer())
-    const ext = path.extname(file.name).toLowerCase()
+    // Resolve extension from file name, falling back to MIME type (e.g. clipboard pastes)
+    let ext = path.extname(file.name).toLowerCase()
     if (!['.jpg', '.jpeg', '.png', '.gif', '.webp'].includes(ext)) {
-      return NextResponse.json({ error: 'Invalid file extension' }, { status: 400 })
+      const mimeToExt: Record<string, string> = {
+        'image/jpeg': '.jpg',
+        'image/png': '.png',
+        'image/gif': '.gif',
+        'image/webp': '.webp',
+      }
+      ext = mimeToExt[file.type] || '.png'
     }
     const filename = `${randomUUID()}${ext}`