fix(review): gutter hover button + bump @pierre/diffs to 1.1.20

[backnotprop]

Summary

• Fix gutter hover button not rendering — renderHoverUtility was calling getHoveredLine() during React render to decide whether to show the button. But Pierre updates hover state imperatively via pointer events (no React re-render), so the button was permanently null. Now the button always renders; getHoveredLine() is only called at click time.
• Fix button styling — matched Pierre's own [data-utility-button] spec: 1lh sizing, --diffs-modified-base background, correct margin/padding. Previously used hardcoded 1.25rem and oklch() purple that didn't match Pierre's theme.
• Bump @pierre/diffs from 1.1.12 → 1.1.20 — picks up split-view scroll fix, WorkerPool race condition fix, hydration fixes, CSS refactoring, and empty-file-as-deleted fix.

Test plan

• Hover over line numbers in split and unified diff — the + button should appear on every line hover
• Click the + button — comment toolbar should open for that line
• Button color should match Pierre's blue theme, not purple
• Button should not clip or overflow the line number cell
• Verify no regressions with hunk expansion, line selection, annotations

[backnotprop]

Note for follow-up work: The <File> component is available from @pierre/diffs/react as of 1.1.20 — it renders a single syntax-highlighted file (no diff). We don't use it in this PR, but it's there for any future work that needs plain file rendering (e.g. viewing unchanged files, full-file context, etc.).