- add `min-w-0` to chat/flex containers to prevent composer/sidebar overflow
- move `BranchToolbar` into the chat column so layout stays aligned with plan sidebar
- extract sidebar inset base classes to shared logic and add a regression test for `min-w-0`

- add shared footer layout breakpoint helper with tests
- switch chat composer footer to compact menu when width is constrained
- preserve plan sidebar toggle behavior while consolidating footer actions

- Replace `SIDEBAR_INSET_BASE_CLASSNAME` with an inline class string in `sidebar.tsx`
- Delete the now-unused `sidebar.logic.ts` helper and its focused unit test

Co-authored-by: codex <codex@users.noreply.github.com>

- Measure composer width from the form element in initial and resize paths
- Avoid relying on ResizeObserver contentRect width for compact footer layout decisions

- Add composer footer/action data attributes for targeted width measurement
- Enforce a minimum composer width before allowing inline diff sidebar