fix: Provider sync on every startup + sandbox fixes (v2.0.4)

Critical fixes:
- ALWAYS sync active AG X provider to endpoints.json on startup
  (previously only synced on first run, causing LS to use stale data)
- Add --no-sandbox to proxy process spawns (fixes SIGTRAP crash on Linux)
- Add --no-sandbox for all Electron modes (not just headless)
- Add --ag-reset flag to re-trigger welcome screen
- Bump version to 2.0.4

Root causes fixed:
1. endpoints.json was never updated after first-run, so LS used wrong provider
2. Electron child processes (translation proxy) crashed due to SUID sandbox
3. No way to re-show provider selection after initial setup

package.json

@@ -1,7 +1,7 @@
 {
   "name": "ag-x",
   "productName": "AG X",
-  "version": "2.0.3",
+  "version": "2.0.4",
   "description": "AG X - Agentic Desktop Application",
   "homepage": "https://ag-x.dev",
   "author": {