fix: hide install button when PWA already installed

- Button hidden by default; async isPwaInstalled() check (standalone mode,
  ponomar-installed flag, getInstalledRelatedApps) unhides only if installable
- Click hint 'already installed' branch uses same multi-signal detection
- manifest id '/' for getInstalledRelatedApps matching on desktop Chrome

vite.config.ts

@@ -42,6 +42,7 @@ export default defineConfig({
         ],
       },
       manifest: {
+        id: '/',
         name: 'Ponomar — Orthodox Calendar',
         short_name: 'Ponomar',
         description: 'Orthodox Church calendar and liturgical information',