feat: split worktree/diff-type into separate controls

[backnotprop]

Summary

• The single dropdown that mixed diff type selection with worktree switching was confusing — entering a worktree silently replaced all options with no indicator of which worktree was active
• Split into two controls: Context dropdown (repo/worktree switcher, only shown when worktrees exist) and View dropdown (always the same diff type options)
• Context dropdown highlights with a primary-colored border when a worktree is active
• Users can now switch directly between worktrees without going "back to main" first

Test plan

• Run ./tests/manual/local/test-worktree-review.sh to launch sandbox with 4 worktrees
• Verify Context dropdown appears above View dropdown listing worktrees with "(worktree)" label
• Select a worktree — pill highlights, files update to that worktree's changes
• Switch View options (Uncommitted/Last commit/vs main) within a worktree context
• Switch directly between worktrees without returning to main
• Switch back to main branch in Context — restores main repo files
• Empty worktree shows appropriate empty state
• Detached HEAD worktree uses directory name as label
• Without worktrees: Context dropdown doesn't render, View dropdown works as before

🤖 Generated with Claude Code

[backnotprop]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

🤖 Generated with Claude Code

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