Open the TestFlight beta and add a feedback form to the website

[whoisaldo]

Summary

Apple approved iPad v0.3.0, build 143, so the site now tells people the beta is open and gives them a place to send bugs and feedback.

• Download page: the iPad card says build 143 passed review, lists the three install steps, and the invite button reads "Join the TestFlight beta". A callout in that card, a "Report a problem" link in the Windows card and a line in the hero all point at the new feedback section.
• Feedback form (download.html#feedback): bug report or general feedback, with what happened, steps (bug only), iPad model, Windows/GPU and connection. Sending opens the visitor's mail app addressed to hello@sideband.studio with the subject ([EternalMonitor bug] … or [EternalMonitor feedback] …) and body filled in and the release versions appended. If no mail app opens, the composed message appears with a copy button. Without JavaScript the form posts to the same mailto address. No third-party form service and no backend.
• Home page: status card and FAQ say the beta is open and link to the form; a new FAQ entry covers where to send feedback.
• Docs: README, SETUP, the installer quick start and docs/beta-testing.md drop the "after Apple approves" wording and mention where feedback goes.
• Footer contact on both pages now uses hello@sideband.studio. These two hunks are identical to the ones in Point the contact email at hello@sideband.studio #54, so either merge order is fine.

Verification

• Windows installer link: downloaded the v0.3.0 asset and its SHA-256 matches the value on the page and in the release notes.
• TestFlight public link: the invite page loads as "Join the EternalMonitor beta" with a View in TestFlight button.
• Headless Chrome at 390, 820 and 1280 px: layout renders with no overflow; screenshots taken before and after a send.
• Scripted form test: steps field hides for General feedback and returns for Bug report; an empty message is blocked by native validation; a filled form shows the fallback panel with the expected To, Subject and body, and the page stays put.
• node --check docs/script.js passes; both pages parse with balanced tags.