fix: admin panel light/dark theme support, improved contrast, and backup confirmation

a) Admin panel now respects the site's light/dark theme toggle via CSS
   custom properties that respond to [data-theme='light'] on <html>.
   All hardcoded dark colors replaced with semantic CSS variables.

b) Improved contrast in dark mode: secondary text changed from #666/#555
   to #a0a0a0, muted text from #555 to #707070. All text is now clearly
   readable against dark backgrounds.

c) Backup system was already implemented in yaml-service.ts (creates
   timestamped backups in content/.backups/ before each save, keeps last
   10 versions). Now the UI shows a confirmation message that a backup
   was created after each successful save.

app/api/admin/settings/route.ts

@@ -36,7 +36,7 @@ export async function PUT(request: Request) {
   try {
     await writeSettingsYaml(settings);
     invalidateConfigCache();
-    return NextResponse.json({ success: true });
+    return NextResponse.json({ success: true, message: 'Saved successfully. Backup of previous version created.' });
   } catch (err) {
     console.error('[Admin] Failed to write settings.yaml:', err);
     return NextResponse.json({ error: 'Failed to save settings' }, { status: 500 });