fix(branch-picker): handle long branch names

[musnows]

Summary

• Truncate long branch names from the middle in the chat branch picker so selected rows and the create-worktree action stay readable.
• Add a hover tooltip for branch rows and the footer action so users can see the full branch/worktree text.
• Cover the middle truncation helper with focused unit tests.

Changes

• Adds a reusable middleEllipsize helper for stable middle truncation with ....
• Keeps full branch values for checkout/create behavior while using shortened display labels.
• Renders the full hover text through a portal so it is not clipped by the branch picker panel.

Tests

• npm run test -- src/renderer/src/lib/middle-ellipsize.test.ts
• npm run typecheck
• git diff --check
• npm run dist:mac:arm64
• Covered local install verification after packaging: /Applications/Kun.app launched, http://127.0.0.1:18899/health returned ok, and codesign --verify --deep --strict /Applications/Kun.app passed.