feat(theme): VNClagoon brand theme (navy + cyan) as default, dark-first

Add builtin-vnclagoon theme (cyan #00D4FF accent on navy #0A0E1A, DM Sans body
+ Syne headings, self-hosted OFL fonts); set as default theme policy; default
mode dark. Add VNCmail wordmark SVGs (on-dark/on-light) + wire logo/company via
k8s secret template. Placeholder wordmark — swap official styleguide SVG.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

stores/theme-store.ts

@@ -78,8 +78,8 @@ let themeSyncPromise: Promise<void> | null = null;
 export const useThemeStore = create<ThemeState>()(
   persist(
     (set, get) => ({
-      theme: 'system',
-      resolvedTheme: 'light',
+      theme: 'dark',
+      resolvedTheme: 'dark',
       hydrated: false,
       installedThemes: [...BUILTIN_THEMES],
       activeThemeId: null,