fix(site): upgrade @pierre/diffs to 1.1.0-beta.19 (#22895)

Fixes a race condition in `DiffHunksRenderer` where a stale async
highlight callback overwrites the render cache with an old diff, causing
a hunk count mismatch:

```
DiffHunksRenderer.renderHunks: lineHunk doesn't exist
```

## Root cause

The `DiffHunksRenderer` in `@pierre/diffs@1.0.11` caches highlighted AST
results keyed by diff object reference. When the shiki highlighter isn't
fully loaded, it fires `asyncHighlight(diff)` which captures the current
diff in a closure. If the diff changes before that promise resolves,
`onHighlightSuccess` unconditionally overwrites `renderCache` with the
stale diff/result pair. The subsequent `rerender()` then iterates the
new diff's hunks against the old result's `code.hunks` array, crashing
at an out-of-bounds index.

## Fix

Upgrades `@pierre/diffs` from `1.0.11` to `1.1.0-beta.19`, which
completely refactors the rendering pipeline:

- Replaces the per-hunk `code.hunks[hunkIndex]` lookup with flat
`additionLines`/`deletionLines` arrays indexed directly by line index
- Uses a new `iterateOverDiff` callback pattern instead of the
`renderHunks` method
- The `lineHunk doesn't exist` error is gone from the codebase entirely

The only code change on our side is adapting `extractDiffContent()` in
`FilesChangedPanel.tsx` to the new `ChangeContent`/`ContextContent`
types where `deletions`, `additions`, and `lines` are now counts with
index pointers into top-level
`FileDiffMetadata.deletionLines`/`additionLines` arrays.

site/package.json

@@ -53,7 +53,7 @@
 		"@mui/material": "5.18.0",
 		"@mui/system": "5.18.0",
 		"@mui/x-tree-view": "7.29.10",
-		"@pierre/diffs": "1.0.11",
+		"@pierre/diffs": "1.1.0-beta.19",
 		"@radix-ui/react-avatar": "1.1.11",
 		"@radix-ui/react-checkbox": "1.3.3",
 		"@radix-ui/react-collapsible": "1.1.12",