fix: disk usage shows encrypted data partition, not root

Dashboard System card now reports disk usage for /var/lib/archipelago
(the LUKS encrypted partition) instead of / (small root partition).
This shows the actual usable storage (428GB) rather than the 29GB root.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/archipelago/src/disk_monitor.rs

@@ -33,11 +33,18 @@ fn parse_df_output(stdout: &str) -> Result<(u64, u64, f64)> {
     Ok((used, total, percent))
 }
 
-/// Check disk usage percentage for the root filesystem.
+/// Check disk usage percentage for the data partition.
+/// Uses /var/lib/archipelago (encrypted LUKS partition) if available, falls back to /.
 /// Returns (used_bytes, total_bytes, used_percent).
 pub async fn check_disk_usage() -> Result<(u64, u64, f64)> {
+    // Prefer the encrypted data partition — this is where all user data lives
+    let data_path = if std::path::Path::new("/var/lib/archipelago").exists() {
+        "/var/lib/archipelago"
+    } else {
+        "/"
+    };
     let output = tokio::process::Command::new("df")
-        .args(["--block-size=1", "--output=used,size", "/"])
+        .args(["--block-size=1", "--output=used,size", data_path])
         .output()
         .await
         .context("Failed to run df")?;