feat(web): restore scroll position when browsing files

[brendan-kellam]

Fixes SOU-1260

Summary

Adds scroll position restoration to the code browser's file preview panel (pureCodePreviewPanel.tsx). Previously, scrolling down a file, navigating away, and returning would reset you to the top.

How it works

• The editor grows to its content height, so the actual scroll container is the ScrollArea viewport ([data-radix-scroll-area-viewport]), not CodeMirror's internal scroller. The scroll position is captured from there.
• Position is persisted per file to sessionStorage under a browse-scroll-pos:{repoName}@{revisionName}:{path} key, throttled to one write per animation frame.
• On mount (once the editor view is ready), the saved position is restored via viewport.scrollTo(...) on the next frame so layout has settled.
• Restoration is skipped when a highlight_range query param is present, so the existing highlight scroll-into-view behavior takes precedence.
• A ref guards against re-restoring after the user has scrolled, while still re-running when the file/revision changes.

Notes

• Uses sessionStorage, so positions persist across in-session back/forward and re-navigation but reset when the tab closes.

🤖 Generated with Claude Code

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