fix(ui): timezone picker uses folds SettingsSelect (native-cinny audit 7/N)

Replaced the last raw native <select> (Profile timezone, colorScheme:'dark')
with SettingsSelect. Added an optional `disabled` prop to SettingsSelect for
the saving state. handleSubmit reads the `timezone` state (not the native form
field) so submission is unaffected; the now-unused handleSelectChange was
removed. No raw <select> elements remain in the settings UI.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/components/settings-select/SettingsSelect.tsx

@@ -18,11 +18,13 @@ export function SettingsSelect<T extends string>({
   value,
   options,
   onChange,
+  disabled,
   'aria-label': ariaLabel,
 }: {
   value: T;
   options: SettingsSelectOption<T>[];
   onChange: (v: T) => void;
+  disabled?: boolean;
   'aria-label'?: string;
 }) {
   const [menuCords, setMenuCords] = useState<RectCords>();
@@ -47,6 +49,7 @@ export function SettingsSelect<T extends string>({
         radii="300"
         after={<Icon size="300" src={Icons.ChevronBottom} />}
         onClick={handleMenu}
+        disabled={disabled}
         aria-label={ariaLabel}
         aria-haspopup="menu"
         aria-expanded={!!menuCords}