Install links for websites: frame-control://install

A site can link to frame-control://install?manifest=URL (or ?url=URL) to
install a title with Frame Control. Manifests use FrameDrop's format, so
framedrop.install/v1 is accepted as well as frame-control.install/v1.

- app/install-link.js parses links; main.js registers the scheme (plus
  electron-builder protocols for Info.plist and the .desktop file), takes
  links from open-url, second-instance argv and the first argv, and holds
  them until the page asks for them through preload's onInstallLink.
- ui/frame_webinstall.py checks the URLs (HTTPS only; localhost over http
  only when the link itself is local; no userinfo; every address public,
  rechecked on redirects and pinned for the connection), reads the
  manifest, downloads with a size cap and sha256 check, and dispatch()
  sends .apk to frame_android and .zip/.exe to frame_titles when present.
- server.py adds /api/webinstall/check, start, job and cancel behind the
  existing Host and X-Frame-UI guards; a start needs a one-time id from
  check. Downloads stop on cancel and on shutdown, and leftovers from a
  killed server are swept by PID.
- index.html asks before anything downloads (name, source host, file,
  type, size, whether a sha256 was given) and shows progress.
- docs/web-install.md, docs/install.html (landing page, unpublished).

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

app/package.json

@@ -21,6 +21,14 @@
   "build": {
     "appId": "com.saphid.frame-control",
     "productName": "Frame Control",
+    "protocols": [
+      {
+        "name": "Frame Control install link",
+        "schemes": [
+          "frame-control"
+        ]
+      }
+    ],
     "directories": {
       "output": "dist",
       "buildResources": "build"
@@ -28,6 +36,7 @@
     "files": [
       "main.js",
       "preload.js",
+      "install-link.js",
       "package.json",
       "build/icon.png"
     ],