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/streaming.md

@@ -38,6 +38,9 @@ flat 2D desktop streaming into a window on the Frame's Linux desktop.
 | Immersed / Virtual Desktop | Vendor apps | Immersed has a Mac agent but no known Frame client. Virtual Desktop's developer said he'd "try" to port it ([NewsBreak](https://www.newsbreak.com/news/4892834783961-virtual-desktop-dev-says-he-ll-try-to-bring-the-app-to-steam-frame)). | Not available as of 2026-09-25. Check again later. |
 | WiVRn / ALVR | VR streaming from a Linux or Windows PC | Irrelevant for a Mac host (no SteamVR/OpenXR runtime on macOS) | N/A |
 
+For **VR video files** (180°/360° stereo), don't stream the Mac's screen. Play
+them on the Frame in DeoVR instead: see [vr-video.md](vr-video.md).
+
 ### Pre-seeding the Remmina profile (no typing in the headset)
 
 `scripts/install-apps.sh remmina --vnc-host <your-mac>.local` writes