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,
+}

src/application/ports/favorites_ports.rs

@@ -1,4 +1,4 @@
-use crate::application::dtos::favorites_dto::FavoriteItemDto;
+use crate::application::dtos::favorites_dto::{BatchFavoritesResult, FavoriteItemDto};
 use crate::common::errors::Result;
 use async_trait::async_trait;
 
@@ -21,6 +21,14 @@ pub trait FavoritesUseCase: Send + Sync {
 
     /// Check if an item is in user's favorites
     async fn is_favorite(&self, user_id: &str, item_id: &str, item_type: &str) -> Result<bool>;
+
+    /// Add multiple items to favorites in a single transaction.
+    /// Returns enriched favourites list so the client can replace its cache.
+    async fn batch_add_to_favorites(
+        &self,
+        user_id: &str,
+        items: &[(String, String)],
+    ) -> Result<BatchFavoritesResult>;
 }
 
 // ─────────────────────────────────────────────────────
@@ -45,4 +53,12 @@ pub trait FavoritesRepositoryPort: Send + Sync + 'static {
 
     /// Checks if an item is in favorites.
     async fn is_favorite(&self, user_id: &str, item_id: &str, item_type: &str) -> Result<bool>;
+
+    /// Insert multiple items in a single transaction.
+    /// Returns the number of rows actually inserted (ignoring duplicates).
+    async fn add_favorites_batch(
+        &self,
+        user_id: &str,
+        items: &[(String, String)],
+    ) -> Result<u64>;
 }

src/application/services/favorites_service.rs

@@ -1,4 +1,4 @@
-use crate::application::dtos::favorites_dto::FavoriteItemDto;
+use crate::application::dtos::favorites_dto::{BatchFavoritesResult, BatchFavoritesStats, FavoriteItemDto};
 use crate::application::ports::favorites_ports::{FavoritesRepositoryPort, FavoritesUseCase};
 use crate::common::errors::{DomainError, ErrorKind, Result};
 use async_trait::async_trait;
@@ -94,4 +94,51 @@ impl FavoritesUseCase for FavoritesService {
         );
         self.repo.is_favorite(user_id, item_id, item_type).await
     }
+
+    async fn batch_add_to_favorites(
+        &self,
+        user_id: &str,
+        items: &[(String, String)],
+    ) -> Result<BatchFavoritesResult> {
+        info!(
+            "Batch adding {} items to favorites for user {}",
+            items.len(),
+            user_id
+        );
+
+        // Validate all item types
+        for (item_id, item_type) in items {
+            if item_type != "file" && item_type != "folder" {
+                return Err(DomainError::new(
+                    ErrorKind::InvalidInput,
+                    "Favorites",
+                    format!(
+                        "Item type must be 'file' or 'folder' for item '{}'",
+                        item_id
+                    ),
+                ));
+            }
+        }
+
+        let requested = items.len();
+        let inserted = self.repo.add_favorites_batch(user_id, items).await?;
+        let already_existed = requested as u64 - inserted;
+
+        info!(
+            "Batch favorites for user {}: {} requested, {} inserted, {} already existed",
+            user_id, requested, inserted, already_existed
+        );
+
+        // Return the full enriched list so the client can replace its cache
+        let favorites = self.repo.get_favorites(user_id).await?;
+
+        Ok(BatchFavoritesResult {
+            stats: BatchFavoritesStats {
+                requested,
+                inserted,
+                already_existed,
+            },
+            favorites,
+        })
+    }
 }