Refine offline mode UX for unavailable tracks and playlist filters

- Remove wifi-off icon from unavailable tracks in playlist detail view,
  keeping only the disabled styling for a cleaner look
- Rename "Available Offline" filter to "Fully Available Offline" to
  clarify it shows only playlists where all tracks are available
- Update filter logic to only show all_local playlists in the fully
  available filter, not partially available ones
- Fix visible filter in offline mode to respect showPartialPlaylists
  setting by using isPlaylistAvailableOffline function

src/lib/i18n/locales/en.json

@@ -482,7 +482,7 @@
     "startWithoutLogin": "Start in offline mode without logging in",
     "noNetworkToast": "No internet connection available",
     "checkingNetwork": "Checking network connection...",
-    "available": "Available Offline",
+    "available": "Fully Available Offline",
     "partiallyAvailable": "Partially Available",
     "notAvailableOffline": "Not Available Offline",
     "viewOnly": "View Only (Offline)",