dashboard: per-player settings, storage usage; drop Config tab

- Remove the separate Config tab (3 tabs now); each player's settings
  (content type, mount point, music folder, fat order, sync command) are shown
  in a PLAYER SETTINGS card on the dashboard
- Show mounted storage on the device card: total/free space (statvfs) plus a
  usage gauge
- Add library + sync-pipeline cards and full-width SYNC PATH chain
- Update keybar/help/README for 3 tabs

Cargo.lock

@@ -258,6 +258,7 @@ dependencies = [
  "anyhow",
  "chrono",
  "dirs",
+ "libc",
  "ratatui",
  "serde",
  "toml",