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

src/disk.rs

@@ -0,0 +1,50 @@
+use std::path::Path;
+
+/// Total and available bytes on a mounted filesystem (statvfs).
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct DiskStats {
+    pub total: u64,
+    pub available: u64,
+}
+
+pub fn disk_stats(path: &Path) -> Option<DiskStats> {
+    let c = std::ffi::CString::new(path.to_str()?).ok()?;
+    let mut s: libc::statvfs = unsafe { std::mem::zeroed() };
+    if unsafe { libc::statvfs(c.as_ptr(), &mut s) } != 0 {
+        return None;
+    }
+    let frsize = (s.f_frsize.max(1)) as u64;
+    Some(DiskStats {
+        total: s.f_blocks as u64 * frsize,
+        available: s.f_bavail as u64 * frsize,
+    })
+}
+
+pub fn fmt_bytes(n: u64) -> String {
+    const UNITS: [&str; 5] = ["B", "KiB", "MiB", "GiB", "TiB"];
+    let mut v = n as f64;
+    let mut u = 0;
+    while v >= 1024.0 && u < 4 {
+        v /= 1024.0;
+        u += 1;
+    }
+    if u == 0 {
+        format!("{v:.0} {}", UNITS[u])
+    } else {
+        format!("{v:.1} {}", UNITS[u])
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::fmt_bytes;
+
+    #[test]
+    fn formats_bytes() {
+        assert_eq!(fmt_bytes(0), "0 B");
+        assert_eq!(fmt_bytes(512), "512 B");
+        assert_eq!(fmt_bytes(1024), "1.0 KiB");
+        assert_eq!(fmt_bytes(137438953472), "128.0 GiB");
+        assert_eq!(fmt_bytes(1_099_511_627_776), "1.0 TiB");
+    }
+}