feat(settings): add timezone field to General tab

- New Timezone section with text input + Detect button
- Detect auto-fills from browser Intl API
- Save calls PUT /api/settings (which now propagates to scheduler)
- Briefing tab firing timezone hint reads stored value instead of live browser API

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

frontend/src/views/SettingsView.vue

@@ -15,6 +15,27 @@ const toastStore = useToastStore();
 const pushStore = usePushStore();
 const assistantName = ref("");
 const defaultModel = ref("");
+const userTimezone = ref("");
+const savingTimezone = ref(false);
+const timezoneSaved = ref(false);
+
+function detectTimezone() {
+  userTimezone.value = Intl.DateTimeFormat().resolvedOptions().timeZone;
+}
+
+async function saveTimezone() {
+  savingTimezone.value = true;
+  timezoneSaved.value = false;
+  try {
+    await apiPut('/api/settings', { user_timezone: userTimezone.value });
+    timezoneSaved.value = true;
+    setTimeout(() => (timezoneSaved.value = false), 2000);
+  } catch {
+    toastStore.show('Failed to save timezone', 'error');
+  } finally {
+    savingTimezone.value = false;
+  }
+}
 const backgroundModel = ref("");
 const installedModels = ref<string[]>([]);
 const defaultChatModel = ref("");
@@ -710,6 +731,7 @@ onMounted(async () => {
   const allSettings = await apiGet<Record<string, string>>("/api/settings");
   defaultModel.value = allSettings.default_model ?? "";
   backgroundModel.value = allSettings.background_model ?? "";
+  userTimezone.value = allSettings.user_timezone ?? "";
   chatRetentionDays.value = allSettings.chat_retention_days !== undefined
     ? Number(allSettings.chat_retention_days)
     : 90;
@@ -1463,6 +1485,32 @@ function formatUserDate(iso: string): string {
         </div>
       </section>
 
+      <!-- Timezone -->
+      <section class="settings-section full-width">
+        <h2>Timezone</h2>
+        <p class="section-desc">Used to schedule briefings and format times in chat. Set this to your local IANA timezone (e.g. America/New_York, Europe/London).</p>
+        <div class="field">
+          <label for="user-timezone">Your timezone</label>
+          <div style="display:flex; gap:0.5rem; align-items:center">
+            <input
+              id="user-timezone"
+              v-model="userTimezone"
+              type="text"
+              class="input"
+              placeholder="e.g. America/New_York"
+            />
+            <button class="btn-secondary" type="button" @click="detectTimezone">Detect</button>
+          </div>
+          <p class="field-hint">Click Detect to auto-fill from your browser.</p>
+        </div>
+        <div class="actions">
+          <button class="btn-save" @click="saveTimezone" :disabled="savingTimezone">
+            {{ savingTimezone ? 'Saving…' : 'Save' }}
+          </button>
+          <span v-if="timezoneSaved" class="saved-msg">Saved!</span>
+        </div>
+      </section>
+
       <!-- Model Management -->
       <section class="settings-section full-width">
         <div class="model-mgmt-header">
@@ -2104,7 +2152,7 @@ function formatUserDate(iso: string): string {
           </div>
         </div>
         <p class="field-hint" style="margin-top: 0.5rem">
-          Firing in timezone: <strong>{{ Intl.DateTimeFormat().resolvedOptions().timeZone }}</strong>
+          Firing in timezone: <strong>{{ userTimezone || 'UTC (not configured — set in General settings)' }}</strong>
         </p>
       </section>