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/locales/en.json

@@ -1524,7 +1524,8 @@
     "welcome": "I'm here to help you synthesize your notes, generate new ideas, or discuss your notebooks.",
     "searching": "Searching...",
     "noNotesFoundForContext": "No relevant notes found for this question. Answer with your general knowledge.",
-    "webSearch": "Web Search"
+    "webSearch": "Web Search",
+    "timeoutWarning": "Response is taking longer than expected..."
   },
   "labHeader": {
     "title": "The Lab",