feat(theme): per-theme brand logos (VNClagoon wordmark ↔ SRC mark)

Add optional logoLightUrl/logoDarkUrl to InstalledTheme + resolveThemeLogo()
helper; set logos on vnclagoon + src themes; login page and nav-rail prefer the
active theme's logo, falling back to the global config logo. So switching theme
switches the whole brand. 0 type errors.

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

lib/plugin-types.ts

@@ -204,6 +204,13 @@ export interface InstalledTheme {
    * lifecycle as `css` to keep localStorage small.
    */
   skin?: string;
+  /**
+   * Optional brand logos. When this theme is the active theme, these override
+   * the global login/app logo (a full brand skin, not just colours). Falls back
+   * to the configured global logo when unset.
+   */
+  logoLightUrl?: string;
+  logoDarkUrl?: string;
   variants: ThemeVariant[];
   enabled: boolean;
   builtIn: boolean;