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/fileViewer.js

@@ -294,17 +294,11 @@ class FileViewer {
   }
   
   /**
-   * Check if a MIME type is text-viewable
+   * Check if a MIME type is text-viewable — delegates to the single global
+   * definition exposed by app.js (window.isTextViewable).
    */
   isTextViewable(mimeType) {
-    if (!mimeType) return false;
-    if (mimeType.startsWith('text/')) return true;
-    const textTypes = [
-      'application/json', 'application/xml', 'application/javascript',
-      'application/x-sh', 'application/x-yaml', 'application/toml',
-      'application/x-toml', 'application/sql',
-    ];
-    return textTypes.includes(mimeType);
+    return window.isTextViewable ? window.isTextViewable(mimeType) : false;
   }
   
   /**