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/TaskCard.vue

@@ -1,6 +1,6 @@
 <template>
     <div
-        class="cursor-pointer rounded-lg border border-neutral-200 bg-white p-3 shadow-sm transition hover:shadow-md dark:border-dark-border dark:bg-dark-card"
+        class="cursor-pointer rounded-lg border border-neutral-200 bg-white p-3 shadow-sm transition hover:shadow-md"
         draggable="true"
         @dragstart="onDragStart"
         @dragend="onDragEnd"
@@ -27,7 +27,7 @@
                         :title="$t('clientTicket.linkedTooltip', { number: 'CT-' + String(task.clientTicket.number).padStart(3, '0') })"
                     />
                 </div>
-                <h4 class="text-sm font-semibold text-neutral-900 dark:text-neutral-100">{{ task.title }}</h4>
+                <h4 class="text-sm font-semibold text-neutral-900">{{ task.title }}</h4>
             </div>
             <button
                 class="shrink-0 transition-colors"
@@ -85,7 +85,7 @@
             />
             <span
                 v-else
-                class="ml-auto flex h-5 w-5 items-center justify-center rounded-full bg-neutral-200 text-neutral-400 dark:bg-dark-hover dark:text-neutral-400"
+                class="ml-auto 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>