Review UI is not usable on mobile-sized viewports

## Summary

The `plannotator review` browser UI is not usable on a mobile-sized viewport. It appears to keep the desktop split-pane layout instead of switching to a single-column or drawer-based mobile layout.

## Environment

- Plannotator: `0.21.4`
- Command: `plannotator review`
- Tested viewport: `390x844` px
- Browser: Codex in-app browser
- OS: macOS

## What I saw

On the release/onboarding modal:

- Content is visibly clipped/narrow at phone width.
- The modal content does not adapt cleanly to the available width.

After dismissing the modal:

- The review UI stays split into a file sidebar and a diff pane.
- The file sidebar takes about `255px` of a `390px` viewport.
- The diff pane is squeezed to about `134px`, making code review impractical.
- Some top controls extend beyond the viewport; measured examples reached about `491px` on a `390px` viewport.
- The file filter/dropdown menu also renders poorly: long descriptions wrap into very narrow columns, and the bottom `Done` action is partially clipped at the viewport edge.

## Expected behavior

On phone-sized viewports, the review UI should switch to a mobile-specific layout, for example:

- Single-column view with file tree and diff shown as separate modes/screens.
- File list available through a drawer or sheet.
- Diff pane given the full viewport width when reading/reviewing a file.
- Header controls wrapped, collapsed, or moved into an overflow menu.
- Menus and dialogs constrained to the viewport with fully reachable actions.

## Notes

The page document itself did not expose horizontal page scroll in the test, but important controls and inner containers were still clipped or rendered offscreen, so this is likely an internal layout/container issue rather than only `body` overflow.

Start with the browser UI launched by `plannotator review` and reproduce the issue at 390x844px. Trace the release/onboarding modal, split-pane review layout, file filter menu, and header controls; done means the UI adapts to phone-sized viewports without clipped content or unreachable actions.

Written by the indexing model from the issue text.