fix(calendar): correct event timezone handling

- Frontend sends user_timezone (IANA, from Intl.DateTimeFormat) with
  every message POST; threaded through route → generation_task → build_context
- System prompt now tells the LLM the user's timezone so it creates
  events with the correct UTC offset (e.g. 15:00+01:00 not 15:00Z)
- Calendar tool guidance updated to require UTC offset in all event
  datetimes
- EventSlideOver: dateFromIso/timeFromIso now use JS Date to convert
  stored UTC times to local time for display; toIso includes local
  timezone offset when saving so the correct UTC time is stored

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

src/fabledassistant/routes/chat.py

@@ -148,6 +148,7 @@ async def send_message_route(conv_id: int):
     think = bool(data.get("think", False))
     rag_project_id = data.get("rag_project_id") or None
     workspace_project_id = data.get("workspace_project_id") or None
+    user_timezone = data.get("user_timezone") or None
     effective_rag_project_id = workspace_project_id or rag_project_id
 
     # Reject if generation already running for this conversation
@@ -184,6 +185,7 @@ async def send_message_route(conv_id: int):
         think=think,
         rag_project_id=effective_rag_project_id,
         workspace_project_id=workspace_project_id,
+        user_timezone=user_timezone,
     ))
 
     return jsonify({