Add the Config view: edit and save the settings, applied without a restart

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

README.md

@@ -26,6 +26,11 @@ A terminal UI to search for movies and series with [Radarr](https://radarr.video
   for releases or start an automatic search from there.
 - **Downloads**: SABnzbd queue and history plus the Radarr/Sonarr queues. Pause/resume, delete,
   retry failed downloads, remove from the arr queue and blocklist.
+- **Config (F7)**: edit everything in the config file from inside the program: service addresses
+  and API keys, the add-form defaults, refresh interval, posters, notifications. *Test connections*
+  checks the values you typed before saving; *Save* writes the file (permissions kept, `0600` for a
+  new file) and applies the changes at once, no restart. Values that come from `ARR_TUI_*`
+  environment variables are greyed out and are never written to the file.
 - **Log (F6)**: 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
@@ -67,7 +72,7 @@ Keep the file private (`chmod 600`); it holds your API keys.
 
 | Key | Action |
 |---|---|
-| `F1`–`F6` | switch view (Movies, Series, Library, Downloads, Calendar, Log) |
+| `F1`–`F7` | switch view (Movies, Series, Library, Downloads, Calendar, Log, Config) |
 | `Tab`, `Shift+Tab` | switch panel within a view |
 | `?` / `F9` | help |
 | `/` | focus search box |