Fix plan composer layout and sidebar flex shrink behavior

- 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`
e9ffd5752bJulius Marminge committed on 3/9/2026, 6:57:27 PM· committed by Julius Marmingeparent24122b1
4 files changedLine totals unavailable