feat(web): add appearance setting (follow system / dark / light) (#253)

* feat(web): add appearance setting (follow system / dark / light)

Add user-facing appearance preference to the settings page, allowing
users to choose between Follow System, Dark, and Light themes. The
preference is persisted to localStorage and takes priority over
automatic detection in the existing theme pipeline.

* fix(web): update theme on cross-tab appearance change

The storage event handler only updated React state without calling
updateScheme(), leaving data-theme and useTheme subscribers stale.

* fix(web): move cross-tab appearance sync to global initializeTheme

The storage event listener was inside useAppearance(), which is only
mounted on the settings page. Other pages never received cross-tab
theme updates. Move the listener into initializeTheme() so all pages
respond to appearance changes from other tabs.

web/src/lib/locales/en.ts

@@ -249,6 +249,10 @@ export default {
   'settings.language.title': 'Language',
   'settings.language.label': 'Language',
   'settings.display.title': 'Display',
+  'settings.display.appearance': 'Appearance',
+  'settings.display.appearance.system': 'Follow System',
+  'settings.display.appearance.dark': 'Dark',
+  'settings.display.appearance.light': 'Light',
   'settings.display.fontSize': 'Font Size',
   'settings.voice.title': 'Voice Assistant',
   'settings.voice.language': 'Voice Language',