Fix shutdown clearing active worktree/tab from wrong layer

[AmethystLiang]

Summary

• Move activeWorktreeId/activeTabId clearing out of shutdownWorktreeTerminals (a terminal-focused store action) into the UI component (WorktreeContextMenu) that triggers shutdown
• The delete flow already handles clearing active state independently in worktrees.ts, so the store action was doing double duty incorrectly
• Remove unused tabIds variable from shutdownWorktreeTerminals
• Fix pre-existing lint issues in both files (interface→type, curly braces, prefer-ternary, prefer-at)
• Extract terminal helper functions to terminal-helpers.ts to stay under max-lines limit

Test plan

• Shutdown a worktree via context menu → verify active worktree is cleared
• Shutdown a non-active worktree → verify active worktree is unchanged
• Delete a worktree → verify active state is still cleared correctly
• Verify no regressions in terminal tab management

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