OR-231: Safe live file refresh

[sox8502]

Summary

• Automatically refresh open file/artifact previews while the page is visible, with focus/reconnect checks and no Reload toolbar button. Keep dirty drafts and reject stale saves using exact SHA-256 versions.
• Finish OR-231's file pane: 40% default panel, bounded 280px explorer, responsive artifact previews, compact file/artifact context menus, and a minimal filename/action header.
• Move Overleaf failures into a focused popover with recovery actions and collapsed settings; skip sync without a token. Remove the temporary error demo.

Verification

• UI production build, typecheck, localization and style checks
• 73 UI tests, including compact-menu sizing and discard/undo regressions
• 9 dev-slot helper tests
• Rust formatting, clippy and locked debug build
• 730 locked Rust tests (2 ignored), release build and telemetry-safety checks
• Live dev-slot API: exact versions, same-size rewrites, stale-save rejection, conditional overwrite, deletion conflicts and HEAD changes
• Four independent Claude reviews with no blockers after the fixes
• Browser smoke: paper preview, no file Reload button, token-free Overleaf setup
• External rewrite/delete/replace of open clean files and artifacts; confirm polling/focus/reconnect updates and unchanged media stays stable
• Dirty-file conflict, conditional overwrite, cancel/discard tab close in Chrome and Safari
• Confirm responsive pane sizing, title truncation and file/artifact context menus at narrow widths
• Authenticated Overleaf sync/retry against a real project

Preview detection uses size/mtime validators (size-only on older servers); save conflict detection uses exact byte hashes. No filesystem watcher or browser-reload draft persistence. Reopening a tab during an in-flight save can conservatively show a conflict; it does not bypass conditional-save protection.

Tracks OR-231. No version bump or release trigger.