[Kamkmgamer]

For pingdotgg#24017 (review copy on my fork before upstreaming).

Problem: the Files aside in the diff view was a fixed w-[min(16rem,40%)] (shrink-0, no resize handle), so file names in nested-folder repos were truncated to the point of being indistinguishable. The same fixed aside existed in two surfaces: the Changes/checkpoint diff (DiffPanel.tsx) and the PR code tab (PullRequestCodeTab.tsx).

Fix: both asides now size from a new useDiffFileTreePanelSize hook that wraps the repo's existing useResizableWidth + RightPanelResizeHandle pattern (the same one the preview panel uses):

• drag the hairline on the panel's left edge to resize; the width persists in localStorage (t3code:diff-file-tree-width) and is shared by both surfaces
• default 288px, 192px floor, capped at the measured row width minus a 360px floor so the diff column beside it always stays readable
• the row is tracked as state (callback ref) because it only mounts when a patch renders, so the cap re-measures when it appears, not just on first mount

The hover tooltip for truncated names from the issue is out of scope for this pass: rows live in @pierre/trees' shadow DOM and there is no per-row title API. Widening the panel covers the readability ask.

Verification: vp run check:changed is green (format, lint, knip, typecheck, and the existing PullRequestCodeTab browser test). Per repo rules this layout fix takes before/after captures rather than a UI test — captures to be added before sending upstream.

GLM-5.3 with the ZCode harness