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

Supersedes #39625 and #40946 after their native-stack branch metadata prevented GitHub from emitting PR CI events.

Validation

• bun typecheck in packages/app
• bun run typecheck:e2e in packages/app
• bun typecheck in packages/desktop
• bun run build in packages/app
• repository-wide typecheck push hook passed
• app unit suite: 636 passed, 13 skipped; existing Arabic i18n parity failure remains for five unrelated keys