Add SABnzbd client and Downloads view (M4)

SABnzbd client (queue, history, pause/resume, delete, retry) with lenient
number decoding and API-key-safe errors. Downloads view polls while
visible, shows queue and history, and supports pause, delete (with
confirmation) and retry. Add dev/ with pinned Radarr/Sonarr/SABnzbd
containers wired together for manual testing.

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

SPECS.md

@@ -6,7 +6,7 @@ A terminal UI to search for movies and series, add them to Radarr / Sonarr, and
 - **TUI library:** [tview](https://github.com/rivo/tview) (on top of tcell)
 - **License:** MIT
 - **Module:** `git.stinnesbeck.com/nils/arr-tui`
-- **Status:** M3 done
+- **Status:** M4 done
 
 ## 1. Goals
 
@@ -106,8 +106,10 @@ Keybindings (initial proposal):
 | `Enter` | Search / open details / confirm |
 | `a` | Add selected result |
 | `m` / `s` | Switch to movies / series search |
-| `p` | Pause/resume SABnzbd |
-| `d` | Delete selected download (with confirm) |
+| `p` | Pause/resume SABnzbd (Downloads view) |
+| `d` | Delete selected download or history entry (with confirm) |
+| `t` | Retry the selected failed download (history) |
+| `h` | Downloads view: switch between queue and history |
 | `i` | Interactive release search for the selected item |
 | `g` | Grab the selected release |
 | `r` | Refresh |
@@ -177,7 +179,7 @@ The file should be created with mode `0600`; warn if it is more permissive. API
 - Unit tests for each client using `httptest.Server` with recorded JSON fixtures.
 - Config parsing / validation tests.
 - UI logic (formatting, state) kept out of tview widgets so it can be tested without a terminal.
-- Manual testing against real Radarr/Sonarr/SABnzbd instances (ideally a docker-compose file for local dev).
+- Manual testing against real Radarr/Sonarr/SABnzbd instances (`dev/up.sh` starts pinned Radarr/Sonarr/SABnzbd containers, see `dev/README.md`).
 
 ## 9. Milestones
 
@@ -185,7 +187,7 @@ The file should be created with mode `0600`; warn if it is more permissive. API
 - [x] **M1** — Radarr client: system status, lookup, quality profiles, root folders, add movie.
 - [x] **M2** — Movie search + add flow in the UI.
 - [x] **M3** — Sonarr client and series search + add flow.
-- [ ] **M4** — SABnzbd client and Downloads view (queue + history) with live updates and pause/resume/delete.
+- [x] **M4** — SABnzbd client and Downloads view (queue + history) with live updates and pause/resume/delete.
 - [ ] **M5** — Radarr/Sonarr queues merged into the Downloads view.
 - [ ] **M6** — Interactive release search (movies, then series seasons/episodes) and grab.
 - [ ] **M7** — Polish: help screen, error handling, README, release builds.