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/services/briefing_scheduler.py

@@ -56,17 +56,22 @@ async def _get_briefing_enabled_users() -> list[tuple[int, str]]:
     import json
     async with async_session() as session:
         result = await session.execute(
-            select(Setting).where(Setting.key == "briefing_config")
+            select(Setting).where(Setting.key.in_(["briefing_config", "user_timezone"]))
         )
         rows = list(result.scalars().all())
 
-    enabled = []
+    by_user: dict[int, dict[str, str]] = {}
     for row in rows:
+        by_user.setdefault(row.user_id, {})[row.key] = row.value or ""
+
+    enabled = []
+    for user_id, settings in by_user.items():
         try:
-            config = json.loads(row.value) if row.value else {}
+            config = json.loads(settings.get("briefing_config", "{}") or "{}")
             if config.get("enabled"):
-                tz = _resolve_timezone(config.get("timezone", "UTC"))
-                enabled.append((row.user_id, tz))
+                tz_str = settings.get("user_timezone") or config.get("timezone", "UTC")
+                tz = _resolve_timezone(tz_str)
+                enabled.append((user_id, tz))
         except Exception:
             pass
     return enabled
@@ -105,13 +110,15 @@ def _remove_user_jobs(user_id: int) -> None:
 
 # ── Public API ────────────────────────────────────────────────────────────────
 
-def update_user_schedule(user_id: int, config: dict) -> None:
+def update_user_schedule(user_id: int, config: dict, tz_override: str | None = None) -> None:
     """
     Called when a user saves their briefing config via the settings UI.
     Live-patches the scheduler — no restart required.
+    tz_override takes priority over any timezone in config.
     """
     if config.get("enabled"):
-        tz = _resolve_timezone(config.get("timezone", "UTC"))
+        tz_str = tz_override or config.get("timezone", "UTC")
+        tz = _resolve_timezone(tz_str)
         _add_user_jobs(user_id, tz)
     else:
         _remove_user_jobs(user_id)