feat(headroom): add proxy lifecycle management + dashboard UI

Build on the optional Headroom Token Saver from Carmelo Campos
(PR: feat: add optional Headroom token saver). Add managed start/stop
of the local headroom proxy from the dashboard, install detection,
status probing, and a simplified Token Saver UI.

- detect headroom CLI + python>=3.10, probe proxy /health
- spawn/stop proxy as a detached, pid-tracked process
- /api/headroom/{status,start,stop} routes, gated local-only in dashboardGuard
- one-click Start/Stop Headroom modal, no manual config needed
- claude<->openai shape conversion for /v1/compress via 9router translators

Thanks to Carmelo Campos (@carmelogunsroses) for the original Headroom integration.

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

src/dashboardGuard.js

@@ -79,6 +79,8 @@ const LOCAL_ONLY_PATHS = [
   "/api/oauth/cursor/auto-import",
   "/api/oauth/kiro/auto-import",
   "/api/auth/reset-password",
+  "/api/headroom/start",
+  "/api/headroom/stop",
 ];
 
 const LOOPBACK_HOSTS = new Set(["localhost", "127.0.0.1", "::1"]);