Announce finished and failed downloads in the background (M10)

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

SPECS.md

@@ -64,6 +64,7 @@ Notes:
   - Global speed / pause state in the status bar. Pause/resume and delete a SABnzbd job.
 - **Status bar / help:** the bottom bar lists the keys that work in the current view or panel; the top right shows the connection state of each service; `?` opens the full key reference.
 - **Posters:** the detail panes show the poster at the top right; the head (title, facts, path) and the plot are one text that flows along the poster's left side and continues at full width below it (text is wrapped by arr-tui itself, with a narrower width for the first lines); on panes too small for the poster, the text simply uses the full width. The poster is drawn with block characters in 24-bit colors: every cell shows several pixels in the best two-color split (`quadrant`: 2×2 pixels, glyphs from the Block Elements range every font has; `sextant`: 2×3 pixels, sharper, glyphs from Symbols for Legacy Computing). `ui.cover_style` is `auto` (default: sextant on kitty, WezTerm, foot, Ghostty, quadrant elsewhere), `quadrant` or `sextant`. JPEG/PNG only, decoded and downscaled in the process, so no terminal graphics protocol is needed. Library items fetch `/MediaCover/<id>/poster-250.jpg` from their Radarr/Sonarr instance with the API key; search results fall back to the provider's `remoteUrl` (fetched without credentials). Posters are cached in memory, failed downloads are not retried for two minutes, and the poster shrinks or disappears when the pane is too short. `ui.covers: false` disables it.
+- **Download notifications:** a background poller (independent of the visible view) reads SABnzbd's history every 15 s and announces downloads that reach `Completed` or `Failed` as a popup and log entry; the first poll only records the current state. `ui.notify_downloads` (default true) and `ui.bell` (default false, rings the terminal bell) control it. Connection errors are not reported here, the indicators and the log already do that.
 - **Notifications and log (F5):** feedback about actions (added, deleted, grabbed, failures) appears as popups in the bottom right that fade away (errors stay longer). Everything is also kept in the Log view; only connection changes of the background polling are logged, not every failed poll.
 
 ### 4.2 v0.2 (done, M8)
@@ -201,7 +202,8 @@ The file should be created with mode `0600`; warn if it is more permissive. API
 - [x] **M7** — Polish: help screen (`?`/F9), per-service connection indicators, README, Makefile release builds, `--version`.
 - [x] **M8** — Library view (movies/series, filters, text filter, monitor toggle, automatic search) and series drill-down (seasons/episodes, monitoring, search, release search).
 - [x] **M9** — Popup notifications, Log view (F5), context-sensitive key hints in the status bar.
-- [ ] **M10+** — ideas: calendar, completion notifications, delete from library, multiple instances.
+- [ ] **M10** — Download completion notifications (done), delete from library, calendar view.
+- [ ] **M11+** — ideas: more library actions (edit quality profile, root folder), history of grabs per title. Multiple instances per service are explicitly out of scope.
 
 ## 10. Open Questions