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)

src/application/dtos/favorites_dto.rs

@@ -78,3 +78,23 @@ impl FavoriteItemDto {
         self
     }
 }
+
+/// Result DTO for batch add-to-favorites.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct BatchFavoritesResult {
+    /// Statistics about the batch operation
+    pub stats: BatchFavoritesStats,
+    /// Full list of the user's favourites (enriched), so the client can
+    /// replace its local cache in a single round-trip.
+    pub favorites: Vec<FavoriteItemDto>,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct BatchFavoritesStats {
+    /// How many items were requested
+    pub requested: usize,
+    /// How many were actually inserted (new)
+    pub inserted: u64,
+    /// How many were already favourites (skipped)
+    pub already_existed: u64,
+}