feat(ui): sidebar toggles in the Ribbon, keyboard chords, and drag-to-hide

[Limdongcheng]

Summary

Hiding or restoring a sidebar required the View menu; nothing on the always-visible chrome showed the current layout or offered a fast path.

The Ribbon task row gains an always-visible toggle pair whose glyphs mirror the window layout — the side band fills while that sidebar is shown, so the buttons double as a live layout indicator. Cmd+B / Cmd+Shift+B (the VS Code convention) bind the existing TogglePrimarySidebar / ToggleSecondarySidebar commands as the fast path. Dragging a sidebar's resize edge well past its minimum width (a 60 px slack, so overshooting a fast resize does not hide it by accident) also hides it, and the status bar names the command that brings it back.

Hidden panels burn one egui auto-id slot and the central workspace renders inside a globally salted scope, so sibling container ids do not shift when a sidebar toggles — widget state (scroll positions, collapses) survives layout changes.

Validation

• cargo pr-check passes on current main (fmt, source sizes, dependency policy, default-frontends build, clippy -D warnings, reference-backend test suite).
• npm run build in docs/ passes.

UI display logic

• New or changed actions go through the command catalog (CommandId + describe) and are searchable in the command palette. (Reuses the registered TogglePrimarySidebar / ToggleSecondarySidebar commands; the chords and Ribbon buttons are new bindings/surfaces for them.)
• Hiding happens only at Ribbon-group level (dataset kind or width budget); transient states disable with a disabled_reason that says how to unblock. (The toggle pair is always visible; its width is budgeted in the task-row compaction estimate.)
• No layout changes from background events; panels are never auto-closed. (Sidebars change only on explicit clicks, chords, or resize drags; the status bar hint names the restore command.)
• New panels or empty regions show an empty state with a next step. (No new panels.)
• Docs updated in docs/ (English and zh-CN) for user-visible behavior. (Shortcut reference, UI overview, and quick tour in both languages.)