fix(sidecar): pin metrics-api-address to ephemeral port

Even with --api-addr set, mycelium also opens an internal JSON-RPC
endpoint on 127.0.0.1:8990 by default (visible in --debug as
\"Starting JSON-RPC server listen_addr=127.0.0.1:8990\"). When a
previous run left an orphan mycelium running as root — which we
can't SIGKILL from a user-level Child::kill_on_drop — the new
instance fails to bind 8990 and exits ~10s after startup, with no
sidecar://exited event because the kill never landed.

Pin --metrics-api-address to a fresh ephemeral port alongside the
other three. Known limitation: stopping the daemon still doesn't
reliably kill the root child; the user has to `sudo pkill mycelium`
between runs. Will be addressed by an elevated-shutdown hook.

src-tauri/src/sidecar.rs

@@ -110,6 +110,12 @@ impl SidecarHandle {
         let api_port = pick_port()?;
         let tcp_port = pick_port_skip(&[api_port])?;
         let quic_port = pick_port_skip(&[api_port, tcp_port])?;
+        // mycelium also opens an internal JSON-RPC / metrics endpoint on
+        // 127.0.0.1:8990 by default; if 8990 is already taken (e.g. by an
+        // orphan from a previous run we couldn't SIGKILL because it ran as
+        // root) the new instance dies a few seconds after start. Pin this
+        // to a fresh ephemeral port too.
+        let metrics_port = pick_port_skip(&[api_port, tcp_port, quic_port])?;
 
         let data_dir = app
             .path()
@@ -126,6 +132,8 @@ impl SidecarHandle {
             tcp_port.to_string(),
             "--quic-listen-port".to_string(),
             quic_port.to_string(),
+            "--metrics-api-address".to_string(),
+            format!("127.0.0.1:{metrics_port}"),
             "--key-file".to_string(),
             key_path.display().to_string(),
         ];
@@ -149,7 +157,7 @@ impl SidecarHandle {
 
         info!(
             ?bin,
-            api_port, tcp_port, quic_port,
+            api_port, tcp_port, quic_port, metrics_port,
             "spawning mycelium sidecar via pkexec"
         );