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- docs/web-install.md, docs/install.html (landing page, unpublished).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>

.github/workflows/checks.yml

@@ -31,7 +31,7 @@ jobs:
       - name: Server tests
         run: python -m unittest discover -s tests -v
       - name: App syntax
-        run: node --check app/main.js && node --check app/build/make-icon.js && node --check app/build/fetch-deps.js && node --check app/preload.js
+        run: node --check app/main.js && node --check app/build/make-icon.js && node --check app/build/fetch-deps.js && node --check app/preload.js && node --check app/install-link.js
 
   # The server runs on each desktop OS the app ships for, on the Python version
   # the app bundles (app/build/fetch-deps.js) and, on Ubuntu, a newer one.