fix(ui): make sidebar hide threshold edge-based

[nmrtist]

Summary

• Fix sidebar resizing so dragging beyond the fixed edge no longer causes the sidebar to grow again.
• Hide a sidebar only when the pointer is released near or beyond the window edge.
• Keep the sidebar open when resizing it to minimum width.
• Show a hide hint while dragging near the edge and allow cancellation by dragging back.
• Add regression coverage for both sidebar directions and update English and Simplified Chinese documentation.

Validation

• cargo pr-check
• npm run build from docs