fix(pinpoint): restore click handler after exiting plan diff

[backnotprop]

Summary

• Bug: Pinpoint mode stops working (clicks do nothing) after entering and exiting Plan Diff view. Drag selection continues to work fine.
• Root cause: The usePinpoint click effect read highlighterRef.current at effect setup time and exited early if null. After exiting plan diff, the Viewer component fully remounts — usePinpoint's click effect ran before the highlighter init effect (line 500), so the ref was still null. The effect bailed without installing the click handler, and nothing triggered a re-run since all dependencies (stable refs, unchanged isActive) remained the same.
• Why drag selection still worked: Drag selection uses web-highlighter's own internal mouseup listener (set up during highlighter.run()), which is independent of the usePinpoint hook.
• Why hover overlay still worked: The mousemove effect in usePinpoint doesn't depend on highlighterRef, so it continued functioning normally.

Fix

Moved highlighterRef.current from effect setup time into the click handler body, so it's resolved lazily at click time when the highlighter is guaranteed to be initialized.

Test plan

• Open a plan with version history (so diff badge appears)
• Switch to Pinpoint input method
• Click the diff badge to enter Plan Diff view
• Exit Plan Diff view (click "Exit Diff" or press Escape)
• Verify pinpoint hover overlay still appears on elements
• Verify clicking an element in pinpoint mode creates an annotation
• Verify drag selection still works after toggling diff on/off
• Verify pinpoint works on initial load (no regression)

🤖 Generated with Claude Code

[backnotprop]

Code review

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

🤖 Generated with Claude Code