Add --update and --check to the install script, and an Updates tab in the game

--update reuses the options an install was made with (saved to install.conf in the work dir) and
builds nothing when the Frame already has the newest release and Retro Rewind pack; --check only
reports. The installed release is stamped as .release-tag beside the game on the Frame.

An install built on the Frame itself (--frame local) also gets a systemd user service the game's
new Updates tab starts: the update runs in the background at low priority, writes each step and
the build percentage to a status file the tab shows, and reopens the game at the end if it was
closed. Progress does not go through Steam notifications: on the Frame, Steam receives them from
steam_notif_daemon but does not display them.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UyuWx5hNrMpf9R78FZFcbA

docs/releases/frame-beta-3.md

@@ -16,6 +16,13 @@ and nothing built from it may be distributed, so there is no ready-built game he
 - **Adaptive resolution** (`[vr] adaptive_resolution`, off by default): lowers the race's eye
   resolution down to 70% while frames fall behind, and raises it again once they keep up.
   Experimental and not yet tried in a race on the Frame. From Wiicompiled_VR-PLUS.
+- **Updating, from the PC or from inside the game.** `steam-frame-install.sh --update` reuses the
+  options the install was made with and builds nothing when the Frame already has the newest
+  release (and Retro Rewind pack); `--check` only reports. An install built on the Frame itself
+  (`--frame local`) gets an **Updates** tab in the settings panel: it checks for a new release and
+  builds it in the background while the game stays open, showing each step and how far the build
+  is. Close the game meanwhile and the update reopens it when done. Steam notifications were tried
+  for this and do not show on the Frame.
 - **`Launcher/frame-diagnostics.sh`** gathers the logs, crash files, settings, SteamVR logs and
   system state from the Frame into one archive, with a summary of how far startup got. Attach it
   to bug reports.
@@ -51,6 +58,10 @@ Run the install command again; you can leave out `--disc`. Dawn's patches did no
 not rebuilt. Most of the game is, since the translator changed as well as the runtime and renderer,
 so expect a longer update than usual. The build now also downloads Mbed TLS. Add `--retro-rewind` to get Retro Rewind too.
 
+frame-beta-2's script has no `--update`, so this one update is the full install command; it saves
+its options, and from then on `steam-frame-install.sh --update` is enough. To update from inside
+the game later, make this install on the Frame itself with `--frame local`.
+
 ## Known issues
 
 Unchanged from frame-beta-2: