Summary

• Adds an expand-down spacer before the first diff hunk when it doesn't start at line 1
• Adds an expand-up spacer after the last diff hunk when it doesn't reach EOF
• Expands 20 lines per click, matching GitHub/GitLab conventions
• Handles pure insertion/deletion edge cases (zero-count hunk gap computation)
• Recomputes hunk headers after expansion to keep line numbers accurate
• Works in both split and unified diff views

Test plan

• 12 new E2E tests in leading-trailing-expand.spec.ts covering visibility, click behavior, commentability in both modes
• Auto-expand-gaps test selectors updated for compatibility with leading/trailing spacers
• All 47 affected tests pass locally

Closes #335
Supersedes #321 (which covered leading spacer only — this adds both leading and trailing)

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