feat(dashboard): add HTTP health probe for cross-container gateway detection

The dashboard's gateway status detection relied solely on local PID checks
(os.kill + /proc), which fails when the gateway runs in a separate container.

Changes:
- web_server.py: Add _probe_gateway_health() that queries the gateway's HTTP
  /health/detailed endpoint when the local PID check fails. Activated by
  setting the GATEWAY_HEALTH_URL env var (e.g. http://gateway:8642/health).
  Falls back to standard PID check when the env var is not set.
- api_server.py: Add GET /health/detailed endpoint that returns full gateway
  state (platforms, gateway_state, active_agents, pid, etc.) without auth.
  The existing GET /health remains unchanged for backwards compatibility.
- StatusPage.tsx: Handle the case where gateway_pid is null but the gateway
  is running remotely, displaying 'Running (remote)' instead of 'PID null'.

Environment variables:
- GATEWAY_HEALTH_URL: URL of the gateway health endpoint (e.g.
  http://gateway-container:8642/health). Unset = local PID check only.
- GATEWAY_HEALTH_TIMEOUT: Probe timeout in seconds (default: 3).

web/src/i18n/en.ts

@@ -80,6 +80,7 @@ export const en: Translations = {
     notRunning: "Not running",
     startFailed: "Start failed",
     pid: "PID",
+    runningRemote: "Running (remote)",
     noneRunning: "None",
     gatewayFailedToStart: "Gateway failed to start",
     lastUpdate: "Last update",

web/src/i18n/types.ts

@@ -83,6 +83,7 @@ export interface Translations {
     notRunning: string;
     startFailed: string;
     pid: string;
+    runningRemote: string;
     noneRunning: string;
     gatewayFailedToStart: string;
     lastUpdate: string;

web/src/i18n/zh.ts

@@ -80,6 +80,7 @@ export const zh: Translations = {
     notRunning: "未运行",
     startFailed: "启动失败",
     pid: "进程",
+    runningRemote: "运行中（远程）",
     noneRunning: "无",
     gatewayFailedToStart: "网关启动失败",
     lastUpdate: "最后更新",

web/src/pages/StatusPage.tsx

@@ -53,7 +53,8 @@ export default function StatusPage() {
   };
 
   function gatewayValue(): string {
-    if (status!.gateway_running) return `${t.status.pid} ${status!.gateway_pid}`;
+    if (status!.gateway_running && status!.gateway_pid) return `${t.status.pid} ${status!.gateway_pid}`;
+    if (status!.gateway_running) return t.status.runningRemote;
     if (status!.gateway_state === "startup_failed") return t.status.startFailed;
     return t.status.notRunning;
   }