fix+feat: SPA routing, theme system, modern UI, setup wizard improvements

Fixes:
- Go server now serves index.html for all non-asset paths (SPA fallback)
- Null guard on data.documents prevents undefined.length TypeError
- App.vue waits for checkStatus() before rendering AppLayout (race condition)
- PWA manifest no longer references missing icon files
- index.html applies theme class before paint to prevent flash

Features:
- Theme store: dark/light toggle + 6 accent color presets + custom hex picker
- ThemeToggle component in header dropdown
- CSS custom properties for accent color (--accent-400/500/600/700)
- Tailwind accent-* color utilities driven by CSS vars
- SetupWizard: 3-step flow, password confirmation, LDAP test button,
  animated toggle, review step, proper error/success states
- Sidebar: loading skeleton, active page highlight, null-safe document list
- Global .input, .btn-primary, .btn-secondary, .card component classes
- Stub GraphQL responses for setup/login/testLdapConnection

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/layout/AppLayout.vue

@@ -1,25 +1,33 @@
 <script setup lang="ts">
 import { ref } from 'vue'
 import Sidebar from './Sidebar.vue'
+import ThemeToggle from './ThemeToggle.vue'
+import { useAppStore } from '@/stores/app'
 
+const app = useAppStore()
 const sidebarOpen = ref(false)
 </script>
 
 <template>
-  <div class="flex h-screen overflow-hidden bg-surface-900 text-slate-100">
+  <div class="flex h-screen overflow-hidden bg-white dark:bg-slate-900 text-slate-900 dark:text-slate-100">
+
     <!-- Mobile overlay -->
     <transition name="fade">
       <div
         v-if="sidebarOpen"
-        class="fixed inset-0 z-20 bg-black/60 lg:hidden"
+        class="fixed inset-0 z-20 bg-black/50 backdrop-blur-sm lg:hidden"
         @click="sidebarOpen = false"
       />
     </transition>
 
-    <!-- Sidebar — slide-over on mobile, static on desktop -->
+    <!-- Sidebar -->
     <aside
       :class="[
-        'fixed inset-y-0 left-0 z-30 w-64 flex-shrink-0 bg-surface-800 border-r border-slate-700 transform transition-transform duration-200',
+        'fixed inset-y-0 left-0 z-30 w-64 flex-shrink-0',
+        'bg-slate-50 dark:bg-slate-800/50',
+        'border-r border-slate-200 dark:border-slate-700/60',
+        'backdrop-blur-md',
+        'transform transition-transform duration-250 ease-in-out',
         'lg:static lg:translate-x-0',
         sidebarOpen ? 'translate-x-0' : '-translate-x-full',
       ]"
@@ -27,20 +35,39 @@ const sidebarOpen = ref(false)
       <Sidebar @close="sidebarOpen = false" />
     </aside>
 
-    <!-- Main content -->
+    <!-- Main area -->
     <div class="flex flex-col flex-1 min-w-0 overflow-hidden">
-      <!-- Mobile top bar -->
-      <header class="flex items-center gap-3 p-3 border-b border-slate-700 lg:hidden">
+
+      <!-- Top bar (always visible on mobile, only shows breadcrumb on desktop) -->
+      <header class="flex items-center gap-2 px-4 py-3 border-b border-slate-200 dark:border-slate-700/60 bg-white/80 dark:bg-slate-900/80 backdrop-blur-md">
+        <!-- Hamburger (mobile only) -->
         <button
-          class="p-1.5 rounded hover:bg-surface-700"
+          class="btn-ghost p-2 -ml-2 lg:hidden"
           aria-label="Open navigation"
           @click="sidebarOpen = true"
         >
           <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
-            <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M4 6h16M4 12h16M4 18h16" />
+            <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M4 6h16M4 12h16M4 18h16"/>
           </svg>
         </button>
-        <span class="font-semibold">Archivum</span>
+
+        <span class="font-semibold text-slate-900 dark:text-white lg:hidden">Archivum</span>
+
+        <div class="flex-1" />
+
+        <!-- Right side controls -->
+        <ThemeToggle />
+
+        <div class="flex items-center gap-1 pl-2 border-l border-slate-200 dark:border-slate-700 ml-1">
+          <span class="text-sm text-slate-500 dark:text-slate-400 hidden sm:block">
+            {{ app.username ?? 'Guest' }}
+          </span>
+          <button
+            v-if="app.token"
+            class="btn-ghost text-xs"
+            @click="app.logout()"
+          >Sign out</button>
+        </div>
       </header>
 
       <main class="flex-1 overflow-auto">