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>

backend/internal/graph/schema.graphql

@@ -22,6 +22,9 @@ type Mutation {
   # First-run setup.
   setup(input: SetupInput!): Boolean!
 
+  # Test an LDAP configuration before saving.
+  testLdapConnection(input: LDAPInput!): LDAPTestResult!
+
   # Authenticate and receive a bearer token.
   login(username: String!, password: String!): String!
 
@@ -54,6 +57,11 @@ type DocumentMeta {
   updatedAt: String!
 }
 
+type LDAPTestResult {
+  success: Boolean!
+  message: String!
+}
+
 type CommitEntry {
   hash:    String!
   author:  String!