fix(admin): wire up default timezone setting in admin settings page

The admin settings timezone dropdown was incomplete: it had no
pre-selection of the current value, was missing consistent styling,
and the form submission did not persist timezone changes.

Changes:
- frontend.go: load default_timezone from system_settings into the
  systemConfig map passed to the template
- admin_settings.templ: match card styling used by the Base URL
  section; pre-select current timezone with selected?= attribute
- sidecar.go: handle default_timezone in UpdateSystemConfiguration
  by writing to system_settings table instead of system_config;
  update HTMX response to include timezone section with current value
- Add selectedAttr() helper for HTMX HTML string response

internal/handlers/sidecar.go

@@ -388,6 +388,23 @@ func (h *SidecarHandler) UpdateSystemConfiguration(c *echo.Context) error {
 
 	// Update each config value
 	for key, value := range req {
+		if key == "default_timezone" {
+			if _, err := time.LoadLocation(value); err != nil {
+				return c.JSON(http.StatusBadRequest, map[string]string{
+					"error": "invalid timezone",
+				})
+			}
+			err := h.db.UpdateSystemSetting(ctx, database.UpdateSystemSettingParams{
+				SettingKey:   "default_timezone",
+				SettingValue: value,
+			})
+			if err != nil {
+				return c.JSON(http.StatusInternalServerError, map[string]string{
+					"error": "failed to update default timezone",
+				})
+			}
+			continue
+		}
 		_, err := h.db.SetSystemConfig(ctx, database.SetSystemConfigParams{
 			Key:       key,
 			Value:     value,
@@ -408,6 +425,12 @@ func (h *SidecarHandler) UpdateSystemConfiguration(c *echo.Context) error {
 			return c.HTML(http.StatusInternalServerError, `<div class="text-red-500">Failed to fetch updated configuration</div>`)
 		}
 
+		defaultTimezone := "UTC"
+		tz, err := h.db.GetSystemTimezone(ctx)
+		if err == nil && tz != "" {
+			defaultTimezone = tz
+		}
+
 		// Render success message with updated form
 		return c.HTML(http.StatusOK, fmt.Sprintf(`
 <div class="mb-4 p-4 rounded-lg" style="background-color: var(--bg-secondary); border: 1px solid var(--accent);">
@@ -437,6 +460,28 @@ func (h *SidecarHandler) UpdateSystemConfiguration(c *echo.Context) error {
 			</button>
 		</div>
 	</div>
+	<div class="mt-8 card p-6 rounded-lg border" style="background-color: var(--bg-secondary); border-color: var(--border);">
+		<h3 class="text-xl font-semibold mb-6" style="color: var(--text-primary)">System Defaults</h3>
+		<div>
+			<label class="block text-sm font-medium mb-2" style="color: var(--text-primary)">Default Timezone</label>
+			<select name="default_timezone" id="default_timezone" class="w-full px-4 py-2 rounded-lg border" style="background-color: var(--bg-primary); color: var(--text-primary); border-color: var(--border);">
+				<option value="UTC"%s>UTC (Coordinated Universal Time)</option>
+				<option value="America/New_York"%s>Eastern Time</option>
+				<option value="America/Chicago"%s>Central Time</option>
+				<option value="America/Denver"%s>Mountain Time</option>
+				<option value="America/Los_Angeles"%s>Pacific Time</option>
+				<option value="America/Phoenix"%s>Mountain Time (no DST)</option>
+				<option value="America/Anchorage"%s>Alaska Time</option>
+				<option value="Pacific/Honolulu"%s>Hawaii Time</option>
+			</select>
+			<p class="text-sm mt-1" style="color: var(--text-secondary)">Default timezone for users who haven't set their own.</p>
+		</div>
+		<div class="mt-6 flex justify-end">
+			<button type="submit" class="btn-primary px-6 py-2 rounded-lg font-medium">
+				Save Settings
+			</button>
+		</div>
+	</div>
 </form>
 
 <div class="mt-8 card p-6 rounded-lg border" style="background-color: var(--bg-secondary); border-color: var(--border);">
@@ -447,7 +492,16 @@ func (h *SidecarHandler) UpdateSystemConfiguration(c *echo.Context) error {
 		<p><strong>Device Sync:</strong> %s/api/sync</p>
 	</div>
 </div>
-`, baseURL.Value, baseURL.Value, baseURL.Value, baseURL.Value))
+`, baseURL.Value,
+			selectedAttr(defaultTimezone, "UTC"),
+			selectedAttr(defaultTimezone, "America/New_York"),
+			selectedAttr(defaultTimezone, "America/Chicago"),
+			selectedAttr(defaultTimezone, "America/Denver"),
+			selectedAttr(defaultTimezone, "America/Los_Angeles"),
+			selectedAttr(defaultTimezone, "America/Phoenix"),
+			selectedAttr(defaultTimezone, "America/Anchorage"),
+			selectedAttr(defaultTimezone, "Pacific/Honolulu"),
+			baseURL.Value, baseURL.Value, baseURL.Value))
 	}
 
 	return c.JSON(http.StatusOK, map[string]string{
@@ -477,3 +531,10 @@ func sanitizeAll(s string, old string, new string) string {
 	}
 	return result
 }
+
+func selectedAttr(current, value string) string {
+	if current == value {
+		return " selected"
+	}
+	return ""
+}

internal/router/frontend.go

@@ -932,7 +932,13 @@ func registerFrontendRoutes(cfg *Config) {
 		}
 
 		systemConfig := map[string]string{
-			"base_url": baseURL,
+			"base_url":         baseURL,
+			"default_timezone": "UTC",
+		}
+
+		defaultTimezone, err := cfg.Queries.GetSystemTimezone(c.Request().Context())
+		if err == nil && defaultTimezone != "" {
+			systemConfig["default_timezone"] = defaultTimezone
 		}
 
 		var buf bytes.Buffer

templates/admin_settings.templ

@@ -51,19 +51,27 @@ templ AdminSettings(user User, systemConfig map[string]string, errorMessage stri
 									</button>
 								</div>
 							</div>
-							<div class="setting-group">
+							<div class="mt-8 card p-6 rounded-lg border" style="background-color: var(--bg-secondary); border-color: var(--border);">
-								<h3>System Defaults</h3>
+								<h3 class="text-xl font-semibold mb-6" style="color: var(--text-primary)">System Defaults</h3>
-								<label for="default_timezone">Default Timezone</label>
+								<div>
-								<select name="default_timezone" id="default_timezone">
+									<label class="block text-sm font-medium mb-2" style="color: var(--text-primary)">Default Timezone</label>
-									<option value="UTC">UTC (Coordinated Universal Time)</option>
+									<select
-									<option value="America/New_York">Eastern Time</option>
+										name="default_timezone"
-									<option value="America/Chicago">Central Time</option>
+										id="default_timezone"
-									<option value="America/Denver">Mountain Time</option>
+										class="w-full px-4 py-2 rounded-lg border"
-									<option value="America/Los_Angeles">Pacific Time</option>
+										style="background-color: var(--bg-primary); color: var(--text-primary); border-color: var(--border);"
-									<option value="America/Phoenix">Mountain Time (no DST)</option>
+									>
-									<option value="America/Anchorage">Alaska Time</option>
+										<option value="UTC" selected?={ systemConfig["default_timezone"] == "UTC" }>UTC (Coordinated Universal Time)</option>
-									<option value="Pacific/Honolulu">Hawaii Time</option>
+										<option value="America/New_York" selected?={ systemConfig["default_timezone"] == "America/New_York" }>Eastern Time</option>
-								</select>
+										<option value="America/Chicago" selected?={ systemConfig["default_timezone"] == "America/Chicago" }>Central Time</option>
+										<option value="America/Denver" selected?={ systemConfig["default_timezone"] == "America/Denver" }>Mountain Time</option>
+										<option value="America/Los_Angeles" selected?={ systemConfig["default_timezone"] == "America/Los_Angeles" }>Pacific Time</option>
+										<option value="America/Phoenix" selected?={ systemConfig["default_timezone"] == "America/Phoenix" }>Mountain Time (no DST)</option>
+										<option value="America/Anchorage" selected?={ systemConfig["default_timezone"] == "America/Anchorage" }>Alaska Time</option>
+										<option value="Pacific/Honolulu" selected?={ systemConfig["default_timezone"] == "Pacific/Honolulu" }>Hawaii Time</option>
+									</select>
+									<p class="text-sm mt-1" style="color: var(--text-secondary)">Default timezone for users who haven't set their own.</p>
+								</div>
 							</div>
 						</form>
 						<div class="mt-8 card p-6 rounded-lg border" style="background-color: var(--bg-secondary); border-color: var(--border);">