fix: auto-collapse lazy-loaded files in PR diff view

[rrbe]

Summary

• Track the user's last-applied collapse/expand intent on the PR diff toggle and reapply it to files GitHub lazy-loads as the user scrolls further down the PR.
• Stabilize the toggle button label so it stays consistent with the user's intent instead of flipping based on the ratio of collapsed files currently in the DOM.
• Reset intent/observer when a fresh button is injected so state doesn't leak across SPA navigations.

Test plan

• Open a large PR, click Collapse all files, scroll down and confirm newly lazy-loaded files render collapsed.
• From the collapsed state, click Expand all files and verify both current and newly-loaded files expand.
• Manually expand a single collapsed file and scroll further — confirm the manually-expanded file stays expanded while new lazy-loaded files still honor the intent.
• Navigate to another PR via client-side nav and confirm the button's label reflects that PR's fresh state (no stale intent carried over).

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