Add PULSE_LXC_CTID env override for LXC CTID detection

Modern Proxmox LXC containers (cgroup v2 + systemd) don't expose the CTID
inside the guest namespace. The auto-detection in DetectLXCCTID() works
for older LXC setups and when hostname is numeric, but fails for most
production containers where users set custom hostnames.

Changes:
- Added PULSE_LXC_CTID environment variable override in router.go:490-495
- Graceful fallback: auto-detect first, then check env var, then show placeholder
- UI already handles missing CTID by showing "pct exec <ctid>" placeholder

This provides a robust solution for thousands of users:
- Stock Proxmox LXC: Shows `pct exec <ctid>` placeholder (user substitutes manually)
- Custom hostname containers: Can set PULSE_LXC_CTID=171 in compose/systemd
- Numeric hostname containers: Auto-detected (backwards compatible)

Related: FirstRunSetup.tsx already has graceful fallback (line 336-339)

internal/api/router.go

@@ -487,8 +487,11 @@ func (r *Router) setupRoutes() {
 				status["bootstrapTokenPath"] = r.bootstrapTokenPath
 				status["isDocker"] = os.Getenv("PULSE_DOCKER") == "true"
 				status["inContainer"] = system.InContainer()
+				// Try auto-detection first, then fall back to env override
 				if ctid := system.DetectLXCCTID(); ctid != "" {
 					status["lxcCtid"] = ctid
+				} else if envCtid := os.Getenv("PULSE_LXC_CTID"); envCtid != "" {
+					status["lxcCtid"] = envCtid
 				}
 				if containerName := system.DetectDockerContainerName(); containerName != "" {
 					status["dockerContainerName"] = containerName