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>

lib/admin/types.ts

@@ -106,7 +106,7 @@ export interface ThemePolicy {
 export const DEFAULT_THEME_POLICY: ThemePolicy = {
   disabledBuiltinThemes: [],
   disabledThemes: [],
-  defaultThemeId: null,
+  defaultThemeId: 'builtin-vnclagoon',
 };
 
 export interface SettingsPolicy {