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/stores/ui.ts

@@ -1,12 +1,19 @@
 export const useUiStore = defineStore('ui', () => {
     const sidebarCollapsed = ref(false)
     const sidebarOpen = ref(false)
+    const darkMode = ref(false)
 
     if (import.meta.client) {
         const saved = localStorage.getItem('ui-sidebar-collapsed')
         if (saved !== null) {
             sidebarCollapsed.value = saved === 'true'
         }
+
+        const savedDark = localStorage.getItem('ui-dark-mode')
+        if (savedDark !== null) {
+            darkMode.value = savedDark === 'true'
+        }
+        applyDarkClass(darkMode.value)
     }
 
     watch(sidebarCollapsed, (val) => {
@@ -15,6 +22,25 @@ export const useUiStore = defineStore('ui', () => {
         }
     })
 
+    watch(darkMode, (val) => {
+        if (import.meta.client) {
+            localStorage.setItem('ui-dark-mode', String(val))
+            applyDarkClass(val)
+        }
+    })
+
+    function applyDarkClass(dark: boolean) {
+        if (dark) {
+            document.documentElement.classList.add('dark')
+        } else {
+            document.documentElement.classList.remove('dark')
+        }
+    }
+
+    function toggleDarkMode() {
+        darkMode.value = !darkMode.value
+    }
+
     function toggleSidebar() {
         sidebarCollapsed.value = !sidebarCollapsed.value
     }
@@ -27,5 +53,5 @@ export const useUiStore = defineStore('ui', () => {
         sidebarOpen.value = false
     }
 
-    return { sidebarCollapsed, sidebarOpen, toggleSidebar, openMobileSidebar, closeMobileSidebar }
+    return { sidebarCollapsed, sidebarOpen, darkMode, toggleSidebar, openMobileSidebar, closeMobileSidebar, toggleDarkMode }
 })