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/components/chat/chat-container.tsx

@@ -40,6 +40,7 @@ export function ChatContainer({ initialConversations, notebooks, webSearchAvaila
     sendMessage,
     status,
     setMessages,
+    stop,
   } = useChat({
     transport,
     onError: (error) => {
@@ -49,6 +50,15 @@ export function ChatContainer({ initialConversations, notebooks, webSearchAvaila
 
   const isLoading = status === 'submitted' || status === 'streaming'
 
+  // Timeout warning: show toast if response takes > 30s
+  useEffect(() => {
+    if (!isLoading) return
+    const timer = setTimeout(() => {
+      toast.warning(t('chat.timeoutWarning') || 'Response is taking longer than expected...')
+    }, 30000)
+    return () => clearTimeout(timer)
+  }, [isLoading, t])
+
   // Sync historyMessages after each completed streaming response
   // so the display doesn't revert to stale history
   useEffect(() => {
@@ -190,6 +200,7 @@ export function ChatContainer({ initialConversations, notebooks, webSearchAvaila
             <ChatInput
               onSend={handleSendMessage}
               isLoading={isLoading}
+              onStop={stop}
               notebooks={notebooks}
               currentNotebookId={selectedNotebook || null}
               webSearchEnabled={webSearchEnabled}