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>

app/preload.js

@@ -1,7 +1,10 @@
 // Lets the page read this computer's clipboard through Electron, so sending it
-// to the Frame needs no pbpaste, PowerShell, xclip or wl-clipboard.
-const { contextBridge, ipcRenderer } = require("electron");
+// to the Frame needs no pbpaste, PowerShell, xclip or wl-clipboard. Also tells
+// the page where a dropped file or folder lives, so a folder can be sideloaded
+// as a title without zipping it (the local server reads it from there).
+const { contextBridge, ipcRenderer, webUtils } = require("electron");
 
 contextBridge.exposeInMainWorld("frameApp", {
   readClipboard: () => ipcRenderer.invoke("clipboard:read"),
+  pathForFile: (file) => { try { return webUtils.getPathForFile(file) || ""; } catch { return ""; } },
 });