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.

src/components/ui/Header.jsx

@@ -559,7 +559,7 @@ const EnhancedMinimalHeader = ({
                     React.createElement('div', { key: 'txt', style: { lineHeight: 1.2, minWidth: 0 } }, [
                         React.createElement('div', { key: 'r1', style: { display: 'flex', alignItems: 'baseline', gap: '7px' } }, [
                             React.createElement('span', { key: 'n', style: { fontSize: '16px', fontWeight: 800, letterSpacing: '-0.3px', color: '#e8e8eb' } }, 'SecureBit'),
-                            React.createElement('span', { key: 'v', style: { fontFamily: MONO, fontSize: '10px', fontWeight: 500, color: '#56565e' } }, 'v4.9.0')
+                            React.createElement('span', { key: 'v', style: { fontFamily: MONO, fontSize: '10px', fontWeight: 500, color: '#56565e' } }, 'v4.9.1')
                         ]),
                         React.createElement('div', { key: 'r2', className: 'hidden sm:block', style: { fontSize: '11px', color: '#6b6b73', fontWeight: 500 } }, 'End-to-end encrypted')
                     ])