refactor: move prune from Settings to Worktrees view

[rrbe]

Summary

• Moved prune functionality from Settings to the Worktrees view, with +New and Prune buttons in a toolbar above the worktree list
• Added bold repo name label at the top of the worktrees sidebar panel
• Replaced inline prune preview with a confirmation modal (ModalShell) that shows candidates before executing
• Improved topbar repo path visibility with stronger text color and border
• Cleaned up dead code: removed unused CSS (.topbar-new-btn, .help-icon-*), orphaned translation keys, and unused phase state property
• Standardized zh-CN translations to use "Worktree" consistently instead of "工作树"
• Removed global button:hover transform effect and per-element transform: none overrides

Test plan

• Open a repo and verify repo name appears bold at the top of the worktrees sidebar
• Verify +New and Prune buttons appear in a toolbar below the repo name
• Click Prune — confirm a modal appears showing loading, then candidates (or "everything clean")
• Confirm prune execution works via the modal's danger button
• Verify the topbar repo path is clearly readable
• Check both light and dark mode appearance
• Switch to Chinese locale and verify "Worktree" terminology is consistent

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