fix: restart backend under systemd and use .sproutclaw agent dir

Restart via systemctl when running as a service, set SPROUTCLAW_CODING_AGENT_DIR
for the RPC subprocess, and add a frontend poll timeout for failed restarts.

Co-authored-by: Cursor <cursoragent@cursor.com>

build.sh

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+OUT="$SCRIPT_DIR/backend/dist/sproutclaw-web-backend"
+
+echo "[build] compiling Go backend -> $OUT"
+cd "$SCRIPT_DIR/backend"
+/usr/local/go/bin/go build -o "$OUT" ./cmd/server
+chmod +x "$OUT"
+echo "[build] done: $OUT"