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>

dev/compose.yaml

@@ -0,0 +1,45 @@
+# Local test instances for arr-tui. Not for real use: fixed dev API keys, bound to localhost.
+#
+#   docker compose -f dev/compose.yaml up -d
+#   ARR_TUI_* env or dev/config.json (see dev/README.md) points the TUI at them.
+#
+# Ports are offset (+10000) so they do not clash with real instances on the same machine.
+x-common: &common
+  restart: unless-stopped
+  # Rootless podman maps container root to the host user, so bind mounts stay writable.
+  user: "0:0"
+  environment: &env
+    TZ: Europe/Berlin
+
+services:
+  radarr:
+    <<: *common
+    image: ghcr.io/home-operations/radarr:6.3.0.10514
+    environment:
+      <<: *env
+      RADARR__AUTH__APIKEY: devradarrapikey00000000000000000
+    ports: ["127.0.0.1:17878:7878"]
+    volumes:
+      - ./data/radarr:/config
+      - ./data/media:/media
+      - ./data/downloads:/downloads
+
+  sonarr:
+    <<: *common
+    image: ghcr.io/home-operations/sonarr:4.0.20
+    environment:
+      <<: *env
+      SONARR__AUTH__APIKEY: devsonarrapikey00000000000000000
+    ports: ["127.0.0.1:18989:8989"]
+    volumes:
+      - ./data/sonarr:/config
+      - ./data/media:/media
+      - ./data/downloads:/downloads
+
+  sabnzbd:
+    <<: *common
+    image: ghcr.io/home-operations/sabnzbd:5.1.0
+    ports: ["127.0.0.1:18080:8080"]
+    volumes:
+      - ./data/sabnzbd:/config
+      - ./data/downloads:/downloads