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/lib/ai/services/semantic-search.service.ts

@@ -121,8 +121,8 @@ export class SemanticSearchService {
 
     // Build Prisma OR clauses for each keyword
     const searchConditions = searchTerms.flatMap(term => [
-      { title: { contains: term } },
-      { content: { contains: term } }
+      { title: { contains: term, mode: 'insensitive' as const } },
+      { content: { contains: term, mode: 'insensitive' as const } }
     ]);
 
     const notes = await prisma.note.findMany({
@@ -145,9 +145,10 @@ export class SemanticSearchService {
       const content = note.content || ''
       const queryLower = query.toLowerCase()
 
-      // Count occurrences
-      const titleMatches = (title.match(new RegExp(queryLower, 'gi')) || []).length
-      const contentMatches = (content.match(new RegExp(queryLower, 'gi')) || []).length
+      // Count occurrences — escape regex special chars to avoid crashes
+      const escaped = queryLower.replace(/[.*+?^${}()|[\]\\]/g, '\\$&')
+      const titleMatches = (title.match(new RegExp(escaped, 'gi')) || []).length
+      const contentMatches = (content.match(new RegExp(escaped, 'gi')) || []).length
 
       // Boost title matches significantly
       const titlePosition = title.toLowerCase().indexOf(queryLower)