refactor: centralise user timezone as a standalone setting

Browser timezone is now synced to user_settings["user_timezone"] on
every login/page load (App.vue). The briefing scheduler and LLM context
both read from this single source, falling back to the legacy
briefing_config.timezone for existing users during migration.

- App.vue: PUT /api/settings with browser IANA timezone on startAppServices
- routes/chat.py: fall back to stored user_timezone when not sent in request
- briefing_scheduler: read user_timezone setting; briefing_config.timezone
  kept as fallback only
- routes/briefing.py: pass tz_override from user_timezone to live-patched scheduler
- Remove timezone field from BriefingConfig interface and all briefing UI

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

src/fabledassistant/routes/chat.py

@@ -149,6 +149,8 @@ async def send_message_route(conv_id: int):
     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
+    if not user_timezone:
+        user_timezone = await get_setting(uid, "user_timezone") or None
     effective_rag_project_id = workspace_project_id or rag_project_id
 
     # Reject if generation already running for this conversation