refactor: address code-review findings for install detection and diag log

- pwa-install.ts: permanent 'ponomar-ever-installed' marker keeps install
  button hidden on Firefox/Safari after appinstalled clears the offer flag;
  isPwaInstalled moved to core for reuse
- diag-log: self-heal corrupted storage, show MM-DD in tail
- settings: refresh diag panel after clear-cache; uniform completion style
- data-cache.has: plain count(key); document HTML-as-failure intent

src/core/data-cache.ts

@@ -102,7 +102,7 @@ export class DataCache {
       const db = await getDB();
       return await new Promise<boolean | null>((resolve) => {
         const tx = db.transaction(STORE_NAME, 'readonly');
-        const request = tx.objectStore(STORE_NAME).count(IDBKeyRange.only(key));
+        const request = tx.objectStore(STORE_NAME).count(key);
         request.onsuccess = () => resolve(request.result > 0);
         request.onerror = () => resolve(null);
       });