feat(portal): assume the microphone works, retire the mic check from the monitor

[JorimTheuns]

Two changes to the participant portal that landed after #1059 was squash-merged, carried over onto main (which now includes #1063's needs-form link on the event card; the two combine cleanly).

The microphone check is gone

The onboarding deck no longer tests the microphone. Nearly every phone's microphone is fine, and the check cost every participant a permission prompt, a test and a tap before they had said a word. The deck now runs privacy, then a name and tags if the host asks for them, then recording. The permission prompt moves to the first tap on Record.

The safety net is the level meter #1059 introduced: it runs blue, turns yellow when the connection is unhealthy and red when the recording itself is in trouble, and eight seconds of a flat signal (a muted, unplugged or taken-over microphone; a quiet room still returns room tone) opens the microphone test on its own. The one participant in fifty who needs the test still gets it; the other forty-nine never see it.

Retired on both sides

The mic_ok / mic_skipped / mic_blocked funnel stages leave the monitor's vocabulary. The dashboard drops its three mic timeline rows, the drilldown labels and the blocked colour; the server stops accepting them from the portal and stops counting them in the funnel summary. A visitor session written by a portal tab from before the change can still carry one for as long as the session lives; those fold forward to terms, since the participant had passed consent and scanned would read as a step backwards. There is a test for the fold.

The thank you page, settled

• Every button is secondary: "Show QR Code", "Record another conversation", and the footer's "dembrane at your event?", which is full width with its label centred and links to the website's needs form (Portal event invitation links to the website's needs form #1063).
• The heading has more room from the header above it.
• The page fills the viewport, so the footer sits at the bottom even when the content is short.

One column, everywhere

The header keeps to the same 42rem column as the content beneath it. On a wide screen the logo and the actions used to sit at the window's edges, a foot away from the text they belong to; now everything shares one column and the page reads as the phone layout it is. The header's shadow still runs edge to edge.

Verification

• Walked the onboarding deck on a project with nothing to ask: "I understand" lands straight on the recording screen, no mic card.
• Finish page checked at 375px and 1280px: header, content and footer span the same column; footer at the viewport bottom; button full width.
• Frontend: tsc --noEmit, biome, production build; participant / voice / common / conversation / hooks suites (116 pass; the 5 failures are pre-existing and identical on main).
• Server: ruff check clean; test_visitor_session, test_participant_ping, test_conversation_monitor pass (64 tests) in the devcontainer. ruff format drift in these files predates this PR and CI only runs ruff check.
• i18n catalogs extracted twice and stable.

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