Address review findings in the app, server and tests

- app: startup shell, python and ssh probes run asynchronously so a slow
  shell profile can't freeze the window; PATH comes from the user's real
  login shell and a failed lookup isn't cached; a server that never
  answers is killed; the setup offer runs once per launch, only after the
  UI loads, and decides from HostName alone; connect.sh is started through
  `env ... zsh` so it works whatever the login shell is.
- server: volume validates the level before muting or changing anything.
- Steam: null-safe install-manager fields, http.client errors caught in
  store ratings, price fallback when a sale has no final price.
- tests: server output kept for diagnosis, any startup error retried, and
  captures asserted non-cacheable.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/open-questions.md

@@ -40,7 +40,7 @@ build 20260922.6101926, kernel 6.18, aarch64):
   gets its own SteamVR overlay (`valve.steam.desktopgame.<id>`). Three were
   created side by side with `panel-on-frame.sh`. See [panels.md](panels.md).
 
-Still open: 4, 6, 7, 11 (in-headset connect), 12–17.
+Still open: 4, 6, 7, 11 (in-headset connect), 12–21.
 
 ## Check on the headset (in order)
 
@@ -96,6 +96,11 @@ Still open: 4, 6, 7, 11 (in-headset connect), 12–17.
 20. **F-Droid 2.0** (Compose 1.12): does it run? If so, the catalogue can
     install it instead of 1.17.2.
 
+21. **DeoVR local files:** does DeoVR's file browser show `Videos → VR`
+    (the symlink from `push-vr-video.sh`) or `Z:\home\steamos\Videos\VR`, and do
+    the colour-coded test clips play in 3D (red left eye, cyan right) for both
+    H.264 and H.265? Does the DLNA browser find a server on the Mac?
+
 ## Unconfirmed claims made in these docs
 
 - `/home` and `/etc` persist across Frame OS updates. This is inferred from