feat(ui): add canvas thumbnails to the sidebar

[Limdongcheng]

Summary

• Add live canvas thumbnails to the sidebar, including plots, images, and annotations.
• Preserve canvas aspect ratios and show truncated canvas names with full-name tooltips.
• Keep canvas selection and inline renaming available in thumbnail rows.
• Update the English and Simplified Chinese UI overview.

Validation

• cargo pr-check on macOS arm64.
• npm run build in docs/.
• git diff --check.
• Screenshot harness: canvas thumbnails in light and dark themes, including portrait canvases and long names.

UI display logic

• Canvas selection and renaming retain their existing actions.
• Canvas rows remain visible without background-triggered layout changes.
• Empty canvases show their page preview.
• English and Simplified Chinese documentation updated.