fix: Show inline error on Commit input for empty messages

[Ziinc]

Summary

• Empty commit messages previously only showed a toast, which was easy to miss.
• CommitInput now validates before dispatching: empty/whitespace messages show an inline destructive alert under the textarea (aria-invalid + red border), focus the field, and do not call the commit handlers.
• The error clears as soon as the user starts typing.

Screenshots

Before (empty Message, no error):
Commit input before empty-message error
After clicking Commit with empty message:
Commit input showing inline empty-message error

Test plan

• Unit tests in CommitInput.test.tsx cover empty, whitespace, clear-on-type, successful trim, and commit-and-push empty path
• Screenshot QA (commit-empty-message.spec.tsx) confirms red border + “Enter a commit message.” under Message
• Click Commit with empty message in Review tab — see inline error under the Message field
• Type a message — error clears; Commit succeeds
• Empty Commit and push from the split-button menu shows the same inline error
  _{To show artifacts inline, enable in settings.}

  

[cursor]

Why the change lives in CommitInput

Validation is in the input component (not only performCommit) so empty Commit / Commit and push / Cmd+Enter share one path and the error sits on the field. Toast-only feedback was easy to miss next to the Message box; aria-invalid + destructive border + role="alert" text under the textarea match the dialog error pattern elsewhere. performCommit still toasts as a safety net for non-UI callers.

Screenshot QA: commit-empty-message-01-before / 02-error confirm the red border and “Enter a commit message.” appear under Message after clicking Commit empty.