fix: use neutral glyphs for TestFlight and Google Play buttons

[Copilot]

The TestFlight button showed a generic phone-with-download-arrow glyph that reads as a file download, and the "Coming soon on Google Play" button used a hand-redrawn approximation of the Google Play triangle mark.

Adding an Apple logo is not an option: Apple licenses it only to Apple and authorized resellers, and publishes no public TestFlight badge or icon. Referential plain-text use ("Join the iOS beta on TestFlight") is the compliant path, which the page already uses. Similarly, Google requires the official Play badge to be used unmodified, and only for a live listing.

Changes (site/index.html)

• TestFlight button — download-arrow glyph replaced with a neutral paper-plane (beta flight) motif. Link, wording, Beta badge and footer trademark attribution untouched.
• Google Play button — redrawn Play triangle replaced with a neutral clock, matching the "Coming soon / Soon" semantics. The official Play badge can be dropped in once the listing goes live.
• Both glyphs kept symmetrical: same store-glyph class, 24×24 viewBox, stroke width/caps, and aria-hidden="true".

No copy changed, so the mobile.testflight* / mobile.play / mobile.comingSoon keys in site/assets/i18n/*.json are unaffected.

<!-- before: redrawn Google Play mark -->
<path d="M3.6 2.3 13 12l-9.4 9.7A2 2 0 0 1 3 20.3V3.7c0-.5.2-1 .6-1.4Z…" />

<!-- after: neutral "coming soon" clock -->
<circle cx="12" cy="12" r="9" />
<path d="M12 7v5l3.5 2" />