UI polish: refresh button, negative thumbnail cache, and style fixes

- Add refresh button to browser toolbar to re-fetch current folder
- Cache "no thumbnail" results to avoid repeated slow SMB lookups
- Fix list view fallback icon sizing for files without album art
- Fix view toggle button hover (no background/scale on hover)
- Skip re-render when clicking already-active view mode button

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

media_server/services/thumbnail_service.py

@@ -51,6 +51,9 @@ class ThumbnailService:
         absolute_path = str(file_path.resolve())
         return hashlib.sha256(absolute_path.encode()).hexdigest()
 
+    # Sentinel value indicating "no thumbnail available" is cached
+    NO_THUMBNAIL = b""
+
     @staticmethod
     def get_cached_thumbnail(file_path: Path, size: str) -> Optional[bytes]:
         """Get cached thumbnail if valid.
@@ -60,11 +63,24 @@ class ThumbnailService:
             size: Thumbnail size ("small" or "medium").
 
         Returns:
-            Thumbnail bytes if cached and valid, None otherwise.
+            Thumbnail bytes if cached, empty bytes if cached as "no thumbnail",
+            None if not cached.
         """
         cache_dir = ThumbnailService.get_cache_dir()
         cache_key = ThumbnailService.get_cache_key(file_path)
-        cache_path = cache_dir / cache_key / f"{size}.jpg"
+        cache_folder = cache_dir / cache_key
+        cache_path = cache_folder / f"{size}.jpg"
+        no_thumb_path = cache_folder / ".no_thumbnail"
+
+        # Check negative cache first (no thumbnail available)
+        if no_thumb_path.exists():
+            try:
+                file_mtime = file_path.stat().st_mtime
+                cache_mtime = no_thumb_path.stat().st_mtime
+                if file_mtime <= cache_mtime:
+                    return ThumbnailService.NO_THUMBNAIL
+            except (OSError, PermissionError):
+                pass
 
         if not cache_path.exists():
             return None
@@ -109,6 +125,20 @@ class ThumbnailService:
         except (OSError, PermissionError) as e:
             logger.error(f"Error caching thumbnail: {e}")
 
+    @staticmethod
+    def cache_no_thumbnail(file_path: Path) -> None:
+        """Cache that a file has no thumbnail (negative cache)."""
+        cache_dir = ThumbnailService.get_cache_dir()
+        cache_key = ThumbnailService.get_cache_key(file_path)
+        cache_folder = cache_dir / cache_key
+        cache_folder.mkdir(parents=True, exist_ok=True)
+
+        try:
+            (cache_folder / ".no_thumbnail").touch()
+            logger.debug(f"Cached no-thumbnail for {file_path.name}")
+        except (OSError, PermissionError) as e:
+            logger.error(f"Error caching no-thumbnail marker: {e}")
+
     @staticmethod
     def generate_audio_thumbnail(file_path: Path, size: str) -> Optional[bytes]:
         """Generate thumbnail from audio file (extract album art).
@@ -277,10 +307,10 @@ class ThumbnailService:
         if size not in THUMBNAIL_SIZES:
             size = "medium"
 
-        # Check cache first
+        # Check cache first (returns bytes, empty bytes for negative cache, or None)
         cached = ThumbnailService.get_cached_thumbnail(file_path, size)
-        if cached:
-            return cached
+        if cached is not None:
+            return cached if cached else None
 
         # Generate thumbnail based on file type
         suffix = file_path.suffix.lower()
@@ -299,9 +329,11 @@ class ThumbnailService:
             # Video files - already async
             thumbnail_data = await ThumbnailService.generate_video_thumbnail(file_path, size)
 
-        # Cache if generated successfully
+        # Cache result (positive or negative)
         if thumbnail_data:
             ThumbnailService.cache_thumbnail(file_path, size, thumbnail_data)
+        else:
+            ThumbnailService.cache_no_thumbnail(file_path)
 
         return thumbnail_data