feat: return to citation origin

[tenglvjun]

Summary

• Preserve the reader position when opening a citation reference.
• Add a toolbar back button plus Alt+Left and macOS Command+[ navigation shortcuts.
• Clear stale return points when documents are replaced, reparsed, or destroyed.
• Support grouped citation popovers and add English/Simplified Chinese localization.
• Preserve interactive citations, citation superscripts, figure/table references, and target labels during AI translation.
• Translate surrounding prose and embedded figure descriptions while restoring protected academic labels exactly.
• Invalidate translation caches created before the reference-protection protocol (v7).

Validation

• npm run check
• npm test (1133 tests passed)
• npm run build
• git diff --check

Notes

The Markdown viewer remains read-only; the return point is scoped to the active document view. Translation protection is structural and analyzer-backed; prompt instructions are an additional safeguard.