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>

frontend/src/stores/chat.ts

@@ -324,6 +324,7 @@ export const useChatStore = defineStore("chat", () => {
           think,
           rag_project_id: ragProjectId ?? undefined,
           workspace_project_id: workspaceProjectId ?? undefined,
+          user_timezone: Intl.DateTimeFormat().resolvedOptions().timeZone,
         },
       );
       assistantMessageId = resp.assistant_message_id;