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/task/TaskListItem.vue

@@ -1,6 +1,6 @@
 <template>
     <div
-        class="flex cursor-pointer items-stretch gap-3 rounded-[10px] bg-white px-3 py-2.5 transition-colors hover:shadow-sm sm:px-4 dark:bg-dark-card"
+        class="flex cursor-pointer items-stretch gap-3 rounded-[10px] bg-white px-3 py-2.5 transition-colors hover:shadow-sm sm:px-4"
         :class="selected ? 'ring-2 ring-primary-500' : ''"
         @click="emit('click')"
     >
@@ -30,7 +30,7 @@
                 />
             </div>
             <!-- Row 2: title -->
-            <h4 class="mt-1 text-sm font-semibold text-neutral-900 dark:text-neutral-100">{{ task.title }}</h4>
+            <h4 class="mt-1 text-sm font-semibold text-neutral-900">{{ task.title }}</h4>
             <!-- Row 3: tags + status + deadline/calendar/recurrence -->
             <div class="mt-2 flex flex-wrap items-center gap-1.5">
                 <span
@@ -92,7 +92,7 @@
             />
             <span
                 v-else
-                class="flex h-5 w-5 items-center justify-center rounded-full bg-neutral-200 text-neutral-400 dark:bg-dark-hover"
+                class="flex h-5 w-5 items-center justify-center rounded-full bg-neutral-200 text-neutral-400"
             >
                 <Icon name="mdi:account-outline" size="14" />
             </span>