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>

frontend/src/App.vue

@@ -8,7 +8,7 @@ import { useShortcuts } from "@/composables/useShortcuts";
 import { useAuthStore } from "@/stores/auth";
 import { useChatStore } from "@/stores/chat";
 import { useSettingsStore } from "@/stores/settings";
-import { apiGet } from "@/api/client";
+import { apiGet, apiPut } from "@/api/client";
 
 useTheme();
 
@@ -22,6 +22,8 @@ const { showShortcuts, toggleShortcuts, closeShortcuts } = useShortcuts();
 function startAppServices() {
   chatStore.startStatusPolling();
   settingsStore.fetchSettings();
+  // Sync browser timezone to the server on every login/page load.
+  apiPut("/api/settings", { user_timezone: Intl.DateTimeFormat().resolvedOptions().timeZone }).catch(() => {});
 }
 
 function stopAppServices() {

frontend/src/api/client.ts

@@ -329,7 +329,6 @@ export interface BriefingConfig {
   slots: BriefingSlots;
   notifications: boolean;
   temp_unit: 'C' | 'F';
-  timezone: string;
 }
 
 export interface BriefingFeed {
@@ -364,7 +363,6 @@ const DEFAULT_BRIEFING_CONFIG: BriefingConfig = {
   slots: { compilation: true, morning: true, midday: false, afternoon: false },
   notifications: true,
   temp_unit: 'C',
-  timezone: '',
 };
 
 export async function getBriefingConfig(): Promise<BriefingConfig> {

frontend/src/components/BriefingSetupWizard.vue

@@ -20,7 +20,6 @@ const config = reactive<BriefingConfig>({
   slots: { compilation: true, morning: true, midday: false, afternoon: false },
   notifications: true,
   temp_unit: 'C',
-  timezone: '',
 })
 
 // Step 2 — locations

frontend/src/views/SettingsView.vue

@@ -258,7 +258,6 @@ const briefingConfig = ref<BriefingConfig>({
   slots: { compilation: true, morning: true, midday: false, afternoon: false },
   notifications: true,
   temp_unit: 'C',
-  timezone: '',
 });
 const briefingFeeds = ref<BriefingFeed[]>([]);
 const briefingSaving = ref(false);
@@ -284,10 +283,6 @@ function _parseTopics(raw: unknown): string[] {
 async function loadBriefingTab() {
   briefingConfig.value = await getBriefingConfig();
   briefingFeeds.value = await getBriefingFeeds();
-  // Auto-populate timezone from browser if not already stored.
-  if (!briefingConfig.value.timezone) {
-    briefingConfig.value.timezone = Intl.DateTimeFormat().resolvedOptions().timeZone;
-  }
   const allSettings = await apiGet<Record<string, string>>('/api/settings').catch(() => ({} as Record<string, string>));
   briefingIncludeTopics.value = _parseTopics(allSettings['briefing_include_topics'] ?? '[]');
   briefingExcludeTopics.value = _parseTopics(allSettings['briefing_exclude_topics'] ?? '[]');
@@ -1694,35 +1689,6 @@ function formatUserDate(iso: string): string {
         </div>
       </section>
 
-      <!-- Timezone -->
-      <section class="settings-section full-width">
-        <h2>Timezone</h2>
-        <p class="section-desc">
-          Briefing slots fire at the times below in this timezone.
-          Auto-detected from your browser — override if the server should use a different zone.
-        </p>
-        <div class="briefing-timezone-row">
-          <input
-            type="text"
-            class="input"
-            v-model="briefingConfig.timezone"
-            placeholder="e.g. America/New_York"
-            style="flex: 1"
-          />
-          <button
-            type="button"
-            class="btn-secondary"
-            @click="briefingConfig.timezone = Intl.DateTimeFormat().resolvedOptions().timeZone"
-          >Detect</button>
-        </div>
-        <p class="field-hint">
-          Use an
-          <a href="https://en.wikipedia.org/wiki/List_of_tz_database_time_zones" target="_blank" rel="noopener">IANA timezone name</a>
-          (e.g. <code>Europe/London</code>, <code>America/Chicago</code>).
-          Your browser reports: <strong>{{ Intl.DateTimeFormat().resolvedOptions().timeZone }}</strong>
-        </p>
-      </section>
-
       <!-- Slots -->
       <section class="settings-section full-width">
         <h2>Scheduled Slots</h2>
@@ -1747,8 +1713,8 @@ function formatUserDate(iso: string): string {
             </label>
           </div>
         </div>
-        <p v-if="briefingConfig.timezone" class="field-hint" style="margin-top: 0.5rem">
-          Firing in timezone: <strong>{{ briefingConfig.timezone }}</strong>
+        <p class="field-hint" style="margin-top: 0.5rem">
+          Firing in timezone: <strong>{{ Intl.DateTimeFormat().resolvedOptions().timeZone }}</strong>
         </p>
       </section>
 
@@ -3313,12 +3279,6 @@ FABLE_API_KEY=&lt;your-api-key&gt;</pre>
   flex-wrap: wrap;
   margin-top: 0.5rem;
 }
-.briefing-timezone-row {
-  display: flex;
-  gap: 0.5rem;
-  align-items: center;
-  margin-bottom: 0.5rem;
-}
 .briefing-unit-toggle {
   display: flex;
   gap: 0;