Add CalDAV calendar integration, LLM-suggested tags, and settings refinements

- CalDAV integration: per-user calendar config, create/list/search events
  via caldav library, LLM tools for calendar operations from chat
- LLM-suggested tags: new tag_suggestions service prompts LLM with existing
  tags and note content to suggest 3-5 relevant tags; exposed via API
  endpoints (suggest-tags, append-tag); integrated into editor views
  (suggest button + clickable pills) and chat tool calls (pills in
  ToolCallCard with one-click apply)
- Settings/model UI refinements, generation task improvements

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/views/HomeView.vue

@@ -178,10 +178,9 @@ chatStore.fetchStatus().then(() => {
 
 async function onChatSubmit(payload: {
   content: string;
-  model: string;
   contextNoteId?: number;
 }) {
-  const conv = await chatStore.createConversation("", payload.model);
+  const conv = await chatStore.createConversation();
   await chatStore.fetchConversation(conv.id);
   router.push(`/chat/${conv.id}`);
   await chatStore.sendMessage(payload.content, payload.contextNoteId);