refactor(meetings): retire meeting details v1 and its flag gate #3266

Epic: #1765 · Meetings v2 epic: #1451 · Follows: V2-03 (#2875) · Size: S

Branch: this is the last piece of the epic, after V2 has served all traffic. It is not part of the integration branch's first release (#3249).

Problem. A dark launch with no exit becomes permanent. Once V2 serves every viewer, V1 of the meeting details page and the gate that chooses between them are dead code. This issue is the named checklist for removing them, so the removal is planned work rather than an afterthought.

Preconditions (all must hold; see specs/011-meeting-details-redesign/rollout.md and v2-scaffold.md § Definition of done):

• V2 has served 100% of signed-in traffic through the agreed soak period, with no rollback.
• Anonymous visitors have been moved to V2 deliberately (a code change; the gate hard-codes them to V1 today).
• Every state V1 renders has a V2 equivalent, checked against state-matrix.md, including the not-found and terminal-error paths V1's lookup owns.
• The E5-04 V2 E2E suite (content + robust) is green and binding.

Checklist (one PR):

• Delete apps/lfx-one/src/app/modules/meetings/meeting-join-v1/ and its spec.
• Collapse meeting-details-gate/ so /meetings/:id loads MeetingDetailsPageComponent directly; drop the @defer and the hydrated latch.
• Remove the gate's read of MEETING_V2_ENABLED_FLAG. Leave the flag itself: the composer and other meetings surfaces read it too; it retires only when the last of them has.
• Remove e2e/meetings-v2-flag-gate.spec.ts (or rewrite it as a plain V2 page spec).
• Update docs/architecture/frontend/public-meeting-join.md (it describes V1) and component-architecture.md.
• Nothing under meeting-details-page/ changes in that PR. If it has to, V2 took a dependency on V1 the scaffold layout exists to prevent.