Stacked on and consumes the transition foundation in #111.

- wires semantic presence into navigation drawers/dialogs, dropdowns, the Conversation pane, and comment popovers/sheets - keeps keyboard-driven interactions immediate and passes the app-owned motion policy into the editor - retains closing surfaces only while inert and hidden from accessibility, with reduced-motion coverage

Verification: - bun test (966 passed, 2 skipped, 0 failed) - bun run types - bun run ci - bun run build - relevant Playwright files (25 passed)