fix: center branch icons in session menu

[Lampese]

Related issue

Problem / pressure

The branch row in a session's More actions menu overrides the menu's vertical alignment with items-start. Its branch and copy icons sit above the center of the row.

Summary

Remove that override so the row uses Menu.Item's default centered alignment.

Visual explanation

- <Menu.Item className="items-start" ...>
+ <Menu.Item ...>  // uses the menu's align-items: center

Before / after

Same Storybook menu, viewport, theme and interaction state; left is before, right is after.

Test plan

• Visually compared the same SessionTabBar Storybook menu at the same viewport size before and after the change.
• pnpm format and the changed file's Oxfmt check passed.
• pnpm run docs check passed.
• pnpm check: typechecks, lint, Effect lint and script tests passed. The CLI test roost-session-backend-contract.test.ts timed out in “rejects an edit for an older active goal and rebuilds an invalid derived cache” (30 seconds); the remaining recursive test run was stopped after that failure.
• The subsequent i18n and all import/platform/process/presence/public/writer/inbox boundary checks passed separately.