chore: point iOS store button to the public TestFlight beta

[Copilot]

The iOS entry in the mobile section was a disabled "Coming soon on App Store" placeholder. The app is now distributed as a public beta, so the button links to TestFlight (https://testflight.apple.com/join/BjZ4DWPa) with copy and branding that comply with Apple's trademark/marketing guidelines.

Mobile section (site/index.html)

• Disabled <span class="store-btn store-btn-disabled"> replaced with a real <a target="_blank" rel="noopener"> to the TestFlight join link.
• Label is now "Join the iOS beta on TestFlight" with a Beta badge instead of Soon.
• Hint copy split: Android still pending on Google Play, iOS available as pre-release beta software.

Apple guideline compliance

• Dropped the Apple logo glyph — Apple marks may only be used within official badge artwork, and the "Download on the App Store" badge must not represent a TestFlight build. Replaced with a neutral device/download glyph.
• Copy explicitly frames the build as pre-release beta, avoiding any implication of App Store availability.
• Added the trademark attribution to the footer: "TestFlight is a trademark of Apple Inc., registered in the U.S. and other countries and regions."

i18n

New keys (mobile.testflightTop, mobile.testflight, mobile.beta, mobile.testflightTitle, mobile.hintBeta, footer.appleTm) rather than reusing the old ones, so the 29 untranslated locales fall back to the correct English baseline instead of rendering stale "coming soon" text. The old keys remain in the locale files but are no longer referenced; translations can be added incrementally.

<a class="store-btn" href="https://testflight.apple.com/join/BjZ4DWPa" target="_blank" rel="noopener"
   data-i18n-attr="title:mobile.testflightTitle" title="Join the Endurain iOS beta on TestFlight">
  <span class="store-text">
    <small data-i18n="mobile.testflightTop">Join the iOS beta on</small>
    <strong data-i18n="mobile.testflight">TestFlight</strong>
  </span>
  <span class="beta-badge" data-i18n="mobile.beta">Beta</span>
</a>

Styles (site/assets/css/style.css)

• Added .beta-badge (brand green) alongside the existing .soon-badge, which the Google Play button still uses.