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/vite.config.ts

@@ -8,19 +8,15 @@ export default defineConfig({
     vue(),
     VitePWA({
       registerType: 'autoUpdate',
-      includeAssets: ['favicon.ico', 'apple-touch-icon.png'],
       manifest: {
         name: 'Archivum',
         short_name: 'Archivum',
         description: 'AsciiDoc wiki with Git history',
-        theme_color: '#1e293b',
+        theme_color: '#0f172a',
         background_color: '#0f172a',
         display: 'standalone',
-        icons: [
-          { src: 'icons/icon-192.png', sizes: '192x192', type: 'image/png' },
-          { src: 'icons/icon-512.png', sizes: '512x512', type: 'image/png' },
-          { src: 'icons/icon-512.png', sizes: '512x512', type: 'image/png', purpose: 'any maskable' },
-        ],
+        // Icons are optional; add real PNG files to public/icons/ to enable PWA install.
+        icons: [],
       },
       workbox: {
         globPatterns: ['**/*.{js,css,html,ico,png,svg,woff2}'],