feat: batch favorites endpoint + frontend dedup fixes

Backend:
- Add POST /api/favorites/batch endpoint (single multi-row INSERT)
- Add BatchFavoritesResult/BatchFavoritesStats DTOs
- Add batch methods to ports, service, PG repository
- Transaction-based insert with ON CONFLICT DO NOTHING, chunking at 5000

Frontend:
- Rewrite batchFavorites() to single API call (40 requests → 1)
- Add _replaceCacheFromResponse() to avoid extra GET round-trip
- Centralize formatFileSize, isTextViewable, formatDateTime, formatDateShort
- Remove duplicate icon mapping from app.js
- Fix inconsistent quota defaults (10GB everywhere)

static/js/shared.js

@@ -400,8 +400,6 @@ document.addEventListener('DOMContentLoaded', async () => {
     }
 
     function formatDate(value) {
-        if (!value) return 'N/A';
-        const date = typeof value === 'number' ? new Date(value * 1000) : new Date(value);
-        return date.toLocaleDateString(undefined, { year: 'numeric', month: 'short', day: 'numeric' });
+        return window.formatDateShort ? window.formatDateShort(value) : String(value);
     }
 });