feat(ui) : add dark mode toggle and remove inline dark: classes

- Add dark mode toggle button in top nav
- Add darkMode store with localStorage persistence
- Enable Tailwind class-based dark mode
- Import dark.css global overrides
- Remove inline dark: Tailwind classes (handled by global CSS)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/components/ui/AppTopNav.vue

@@ -19,6 +19,14 @@
         </button>
       </div>
       <div class="ml-auto flex items-center gap-4 text-xl text-white sm:gap-8">
+        <button
+          type="button"
+          class="rounded-md p-1.5 text-white/70 transition-colors hover:bg-primary-600 hover:text-white"
+          :title="ui.darkMode ? 'Mode clair' : 'Mode sombre'"
+          @click="ui.toggleDarkMode()"
+        >
+          <Icon :name="ui.darkMode ? 'mdi:weather-sunny' : 'mdi:weather-night'" size="22" />
+        </button>
         <NotificationBell />
         <div class="group relative flex gap-2 sm:gap-4">
           <UserAvatar v-if="user" :user="user" size="md" class="cursor-pointer" />