fix(#1039): hover-to-expand collapsed sidebar temporarily

[xielixing]

Summary

When the sidebar is collapsed, hovering the collapsed strip temporarily expands the nav panel as a floating overlay — no click required. The panel collapses again when the cursor moves away.

Closes #1039

Changes

• WorkspaceBody.tsx: Added \isHoverExpanded\ state with shared timeout management (200ms expand/collapse delay). Wired \onMouseEnter/\onMouseLeave\ on both the collapsed strip and the nav-area flyout.
• WorkspaceBody.scss: Added .is-hover-expanded\ CSS — positions the nav-area as an absolute flyout with shadow, border-radius, and background. Added macOS-specific left offset (72px for traffic lights). Added \prefers-reduced-motion\ fallback.

Behavior

1. Sidebar is collapsed (user clicked collapse or dragged past threshold)
2. User hovers the collapsed strip → after 200ms, nav panel appears as floating overlay
3. User moves cursor into the flyout → stays open
4. User moves cursor away → after 200ms, flyout collapses
5. User clicks expand button → panel permanently expands, hover state resets

Validation

• \ sc --noEmit\ passes (verified from main repo's web-ui with node_modules)
• No new imports or types added
• Changes follow existing patterns in the file (useState, useCallback, React event handlers)
• \prefers-reduced-motion\ respected