Announce finished and failed downloads in the background (M10)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

README.md

@@ -23,6 +23,10 @@ A terminal UI to search for movies and series with [Radarr](https://radarr.video
 - **Downloads**: SABnzbd queue and history plus the Radarr/Sonarr queues. Pause/resume, delete,
   retry failed downloads, remove from the arr queue and blocklist.
 - **Log (F5)**: actions report through popups that fade away; the log keeps the history.
+- **Download notifications**: a popup (and a log entry) when a SABnzbd download finishes or fails,
+  whichever view is open. SABnzbd's history is checked every 15 seconds; downloads that finished
+  before arr-tui started are not announced. `"ui": {"notify_downloads": false}` turns it off,
+  `"bell": true` also rings the terminal bell.
 - Connection indicators for all three services in the top right; `?` shows every key.
 
 Requires Radarr/Sonarr v3+ API (Radarr 5/6, Sonarr 4) and SABnzbd 4+. Only Usenet releases are
@@ -47,7 +51,7 @@ Settings → General.
   "radarr":  { "url": "http://localhost:7878", "apikey": "...", "default_quality_profile": "HD-1080p", "default_root_folder": "/movies" },
   "sonarr":  { "url": "http://localhost:8989", "apikey": "...", "default_quality_profile": "HD-1080p", "default_root_folder": "/tv" },
   "sabnzbd": { "url": "http://localhost:8080", "apikey": "..." },
-  "ui":      { "refresh_seconds": 3, "covers": true, "cover_style": "auto" }
+  "ui":      { "refresh_seconds": 3, "covers": true, "cover_style": "auto", "notify_downloads": true, "bell": false }
 }
 ```