release: v4.9.1 — redesigned QR scanner, UI fixes, README overhaul

UI / design
- Rework the camera scan modal to the new "Start Secure" design: green
  viewfinder with corner brackets, animated scan line, spinner + live
  frame counter, and a blurred dark backdrop. Keep the Html5Qrcode
  #qr-reader video feed, styled to fill the square viewfinder.
- Fix Advanced (network) settings: the fixed landing header (z-50) was
  covering the panel's close button — raise the embedded overlay to z-60.
- Stack the connection-screen footer buttons ("Download desktop app" /
  "Advanced settings") full-width on mobile and tablet instead of in a row.

Docs
- Rewrite README to follow GitHub best practices: capability-oriented
  Features, How it works, and Security model sections; move release notes
  out of the README and point to CHANGELOG.md. Keep logo and screenshots.

Chore
- Bump version to 4.9.1 (header, package.json, manifest) and rebuild bundles.

meta.json

@@ -1,10 +1,10 @@
 {
-  "version": "1782247921955",
-  "buildVersion": "1782247921955",
-  "appVersion": "4.9.0",
-  "buildTime": "2026-06-23T20:52:01.997Z",
-  "buildId": "1782247921955-b39f9ec",
-  "gitHash": "b39f9ec",
+  "version": "1782329449554",
+  "buildVersion": "1782329449554",
+  "appVersion": "4.9.1",
+  "buildTime": "2026-06-24T19:30:49.591Z",
+  "buildId": "1782329449554-cbf5c5f",
+  "gitHash": "cbf5c5f",
   "generated": true,
-  "generatedAt": "2026-06-23T20:52:01.998Z"
+  "generatedAt": "2026-06-24T19:30:49.593Z"
 }