Sideload Linux and Windows builds as Steam Devkit Games

Dropping a game's .zip, folder or .exe on Send to Frame now adds it to the
headset's Steam library through Valve's SteamOS Devkit title path, with the
runtime picked from the program's header: Windows PE -> Proton Experimental
(steam_play=1), aarch64 ELF -> SteamLinuxRuntime_4-arm64, x86-64 ELF ->
SteamLinuxRuntime_4 (through FEX). Other architectures are refused.

- frame/devkit-utils: Valve's devkit-utils vendored unmodified (MIT,
  steamos-devkit v0.20260925.1), synced to ~/devkit-utils by stamp, bundled in
  the app and compiled in CI.
- ui/frame_titles.py: inspect (safe unzip, ELF/PE classification, launch
  target ranking), install(path, name=None, exe=None, runtime=None,
  progress=None), list, launch, remove, plus a CLI.
- ui/server.py: /api/titles (inspect/install/discard/launch/remove),
  /api/titles/job progress, and an upload mode 'title'.
- ui/index.html: confirm dialog (name, launch target, runtime), install
  progress, and a Sideloaded titles list with Launch and Remove. The app's
  preload passes a dropped folder's path.
- tests and docs/sideloading.md. Device-side behaviour is inferred from
  Valve's source; the headset was offline, so none of it has been checked on
  a Frame yet.

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

.github/workflows/checks.yml

@@ -27,7 +27,10 @@ jobs:
             esac
           done
       - name: Python compiles
-        run: python -m py_compile ui/*.py apk-catalog/*.py frame/android/*.py
+        run: |
+          python -m py_compile ui/*.py apk-catalog/*.py frame/android/*.py
+          # Valve's devkit-utils (vendored; run by the Frame's python3). Most have no .py suffix.
+          python -m py_compile $(find frame/devkit-utils -type f ! -name '*.*' ! -name LICENSE) frame/devkit-utils/devkit_utils/*.py
       - name: Server tests
         run: python -m unittest discover -s tests -v
       - name: App syntax