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.toml

@@ -12,6 +12,7 @@ serde = { version = "1", features = ["derive"] }
 chrono = "0.4"
 unicode-normalization = "0.1"
 dirs = "6"
+libc = "0.2"
 anyhow = "1"
 
 [profile.release]