refactor(app): remove legacy layout

[Brendonovich]

Summary

• make the tabs layout the only application shell
• remove the legacy home, sidebar, settings, transition UI, and rollout assets
• preserve legacy directory and session URLs as redirects into drafts and server-scoped tabs
• keep the tabs-era body marker as a static invariant for shared session UI styling

Stack

• Depends on refactor(app): support only v2 servers #39485

Validation

• bun typecheck in packages/app
• bun run build in packages/app
• bun run test:e2e -- --reporter=line in packages/app (95 passed)
• repository-wide typecheck push hook (33 packages passed)
• full app unit suite: 637 passed, 13 skipped, with the existing Arabic i18n parity failure for five unrelated keys