feat(app): version handshake — GUI restarts a stale running daemon

The daemon outlives the GUI, so after an update an OLD daemon can keep serving
the socket and the new GUI just connects to it (stale code — e.g. the missing
TERM fix). Both binaries are now stamped with the git build id (build.rs):
the daemon reports it in `health.build`, and on connect the bridge compares it
to the GUI's own SPACESH_BUILD; on mismatch it shuts the daemon down and lets
ensure_daemon respawn the bundled (matching) one. No-op for unstamped dev
builds or daemons too old to report a build. Build id is shown in Settings.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/spaceshd/build.rs

@@ -0,0 +1,27 @@
+use std::process::Command;
+
+// Stamp the binary with the current git build id so the GUI can detect a stale
+// running daemon (different code) and restart it. Matches app/src-tauri/build.rs.
+fn main() {
+    println!("cargo:rustc-env=SPACESH_BUILD={}", git_build());
+    println!("cargo:rerun-if-changed=../../.git/HEAD");
+    println!("cargo:rerun-if-changed=../../.git/index");
+}
+
+fn git_build() -> String {
+    let sha = Command::new("git")
+        .args(["rev-parse", "--short=12", "HEAD"])
+        .output()
+        .ok()
+        .filter(|o| o.status.success())
+        .map(|o| String::from_utf8_lossy(&o.stdout).trim().to_string())
+        .filter(|s| !s.is_empty());
+    let Some(sha) = sha else { return "dev".into() };
+    let dirty = Command::new("git")
+        .args(["status", "--porcelain"])
+        .output()
+        .ok()
+        .map(|o| !o.stdout.is_empty())
+        .unwrap_or(false);
+    if dirty { format!("{sha}-dirty") } else { sha }
+}