Avoid rerendering the branch picker while scrolling

[cursor]

What Changed

• update branch-list fade masks directly on the scroll element instead of storing them in React state
• move the required DOM synchronization and initial worktree synchronization effects into named hooks
• preserve pagination, search reset, and top/bottom fade behavior

Why

React Doctor identified the synchronous scroll-fade state update as a cascading-render issue. React Scan confirmed that one branch-list scroll increased BranchToolbarBranchSelector renders from 66 to 76. The optimized scroll path now records 0 parent selector renders; newly virtualized rows still render as expected.

UI Changes

No visual change; this is a branch-picker interaction performance fix.

React Scan recordings are attached to the Cursor run:

• Before: branch_picker_react_scan_before.mp4 (66 → 76 selector renders)
• After: branch_picker_react_scan_after_final.mp4 (Last scroll selector renders: 0)

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after React Scan recordings for the interaction
• I included a video for the interaction change

Validation: vp check, vp run typecheck, 30 branch-toolbar unit tests, and React Doctor performance scan (web findings 114 → 113).

  

Note

Avoid rerendering the branch picker scroll fades while scrolling

Replaces React state-driven scroll fade toggling in BranchToolbarBranchSelector with imperative classList updates on the scrollable DOM node. This eliminates rerenders triggered by scroll events. The logic is extracted into two new hooks: useBranchListScrollBehavior (fade class management, scroll reset, pagination) and useInitialWorktreeBranch (worktree branch auto-selection), with no changes to props or external behavior.

^{Macroscope summarized 477c9aa.}