Stop returning GIF URLs from _best_portrait_url

When all available cover URLs are GIFs (coverUrl, stillCoverUrl, and
screenshots), return empty instead of a GIF URL. HTTPImageFetcher cannot
decode GIFs, so attempting to fetch one always returns null. Returning
empty early avoids unnecessary network requests and confusing log noise.

core/boxart_itch.gd

@@ -174,6 +174,7 @@ func _url_for_layout(kind: LAYOUT, cover_url: String, screenshots: Array) -> Str
 			for url in screenshots:
 				if not _is_gif_url(url):
 					return url
+			# cover_url is already sanitized and non-GIF by the time it reaches here
 			return cover_url
 		LAYOUT.BANNER:
 			# Single screenshot fallback (two-screenshot case handled in get_boxart).
@@ -187,7 +188,8 @@ func _url_for_layout(kind: LAYOUT, cover_url: String, screenshots: Array) -> Str
 
 ## Returns a non-GIF URL suitable for portrait/logo from the game data.
 ## Falls back through: stillCoverUrl → coverUrl (non-GIF) → first non-GIF
-## screenshot.
+## screenshot. Returns empty if only GIFs are available, since
+## HTTPImageFetcher cannot decode GIF images.
 func _best_portrait_url(game: Dictionary, screenshots: Array) -> String:
 	# Prefer stillCoverUrl if it's not a GIF
 	var still: String = game.get("stillCoverUrl", "")
@@ -201,8 +203,9 @@ func _best_portrait_url(game: Dictionary, screenshots: Array) -> String:
 	for url in screenshots:
 		if not _is_gif_url(url):
 			return url
-	# Last resort: use whatever we have (GIF)
-	return still if not still.is_empty() else cover
+	# All available URLs are GIFs — return empty so the caller gets null
+	# instead of attempting a doomed fetch.
+	return ""
 
 
 ## Combines two textures side-by-side into a single banner image.