Add Radarr client (M1)

Shared arr HTTP client with API error handling, plus Radarr lookup,
add movie, search and quality profile / root folder / status endpoints.
Tested against httptest fixtures.

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:** M0 done
+- **Status:** M1 done
 
 ## 1. Goals
 
@@ -181,7 +181,7 @@ The file should be created with mode `0600`; warn if it is more permissive. API
 ## 9. Milestones
 
 - [x] **M0** — Project skeleton: `go mod init`, config loading, empty tview app with tabs and status bar.
-- [ ] **M1** — Radarr client: system status, lookup, quality profiles, root folders, add movie.
+- [x] **M1** — Radarr client: system status, lookup, quality profiles, root folders, add movie.
 - [ ] **M2** — Movie search + add flow in the UI.
 - [ ] **M3** — Sonarr client and series search + add flow.
 - [ ] **M4** — SABnzbd client and Downloads view (queue + history) with live updates and pause/resume/delete.