feat(meetings): add signed-in outsider state with register CTA #2878

Design: https://claude.ai/design/p/c70cdcac-911f-4337-9d87-3daed9d85f58?file=Meeting+Details.dc.html&via=share

Prototype settings panel: time / viewer / privacy / rsvpTracking / pastAccess / myRsvp / recur / via / pageStatus. Its look and feel differs from the current app on purpose — follow it.

Epic: #1765 · Meetings v2 epic: #1451 · Plan ID: E2-02 · Size: S · Depends on: E2-01

Delivery: this ships as part of meeting details V2, side by side with the existing page, behind MEETING_V2_ENABLED_FLAG. Do not modify, refactor or delete the existing meeting details page. V1 stays fully functional and is what untargeted users see. Parent: Meetings v2 epic #1451. Full rules in #1765.
Components: follow the prototype for design, but build it from the existing lfx-* components in app/shared/components/. Extend a wrapper before forking it; create a new component only when the design can't be expressed otherwise, and say so in the PR.

Scope. Explicit state for authenticated, !invited, !organizer. When canRegisterForMeeting() (!invited && !restricted && visibility === 'public'), show Register for Meeting with supporting copy explaining what registration gets you. Registration remains modal-based and authenticated-only (the public register endpoint hard-401s anonymous callers).

Acceptance criteria

• Optimistic flip to the RSVP slot on successful registration preserved.
• Copy states that registering adds you to the guest list and enables RSVP.
• Registration modal reachable by keyboard; validation messages unchanged.

Filed from meeting-details-redesign-implementation-plan.md (revised 2026-09-23).