Merge sideloading, install links and devkit pairing

Combines the three feature branches on bundle-deps. Conflicts in server.py,
index.html, preload.js, README and test_server.py keep both sides. The
web-install downloader now uses urllib's default HTTPS context, so the
bundled CA list from 770f26c applies to it on Windows too.

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

app/preload.js

@@ -2,9 +2,16 @@
 // 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).
+// It also receives frame-control://install links (docs/web-install.md): only
+// what the link asked for, never an install; the page asks the user first.
 const { contextBridge, ipcRenderer, webUtils } = require("electron");
 
 contextBridge.exposeInMainWorld("frameApp", {
   readClipboard: () => ipcRenderer.invoke("clipboard:read"),
   pathForFile: (file) => { try { return webUtils.getPathForFile(file) || ""; } catch { return ""; } },
+  onInstallLink: (cb) => {
+    ipcRenderer.removeAllListeners("install-link");
+    ipcRenderer.on("install-link", (_e, req) => cb({ kind: req.kind, target: req.target }));
+    ipcRenderer.send("install-link:ready");
+  },
 });