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.

manifest.json

@@ -1,5 +1,5 @@
 {
-  "name": "SecureBit.chat v4.9.0 - ECDH + DTLS + SAS",
+  "name": "SecureBit.chat v4.9.1 - ECDH + DTLS + SAS",
   "short_name": "SecureBit",
   "description": "P2P messenger with ECDH + DTLS + SAS security, military-grade cryptography and Lightning Network payments",
   "start_url": "./",