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/theme-logo.ts

@@ -0,0 +1,24 @@
+import type { InstalledTheme } from './plugin-types';
+
+/**
+ * Resolve which logo to show. When the active theme carries brand logos
+ * (logoLightUrl/logoDarkUrl), those win — so switching theme switches the whole
+ * brand (VNClagoon wordmark ↔ SRC mark). Otherwise falls back to the globally
+ * configured logo. Returns a raw path; the caller applies withBasePath().
+ */
+export function resolveThemeLogo(
+  themes: InstalledTheme[],
+  activeThemeId: string | null,
+  isDark: boolean,
+  fallbackLight: string,
+  fallbackDark: string,
+): string {
+  const theme = activeThemeId ? themes.find((t) => t.id === activeThemeId) : undefined;
+  if (theme) {
+    const themed = isDark
+      ? (theme.logoDarkUrl ?? theme.logoLightUrl)
+      : (theme.logoLightUrl ?? theme.logoDarkUrl);
+    if (themed) return themed;
+  }
+  return isDark ? (fallbackDark || fallbackLight) : (fallbackLight || fallbackDark);
+}