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/services/generation_task.py

@@ -151,6 +151,7 @@ async def run_generation(
     think: bool = False,
     rag_project_id: int | None = None,
     workspace_project_id: int | None = None,
+    user_timezone: str | None = None,
 ) -> None:
     """Stream LLM response into buffer with periodic DB flushes."""
     MAX_TOOL_ROUNDS = 5
@@ -183,6 +184,7 @@ async def run_generation(
         excluded_note_ids=excluded_note_ids,
         rag_project_id=rag_project_id,
         workspace_project_id=workspace_project_id,
+        user_timezone=user_timezone,
     ))
 
     messages, context_meta = await context_task