fix(nav-panel): add collapsible chevron indicator to workspace section header

[xielixing]

Fix: Workspace section collapsed/expanded states look identical (#976)

Problem

The workspace section header in the NavPanel uses collapsible mode without onSceneOpen, so the ChevronRight indicator was never rendered. Users could collapse/expand the section but had no visual cue showing the current state — collapsed and expanded headers looked identical.

Additionally, when a user manually collapsed the workspace section and then added a new workspace, the section stayed collapsed, making the new workspace invisible until the user manually expanded it.

Root Cause

In SectionHeader.tsx, the chevron indicator was gated behind onSceneOpen ? (line 63), so collapsible-only sections (workspace, assistant sessions) never showed an indicator.

Changes

1. SectionHeader.tsx — Render the chevron for both onSceneOpen and collapsible sections. Added --collapsed and --expanded CSS modifier classes that rotate the chevron based on isOpen state.

2. NavPanel.scss — Added transform: rotate(0deg) for collapsed state and transform: rotate(90deg) for expanded state, with the existing transition: transform providing smooth animation.

3. MainNav.tsx — Added a useEffect that auto-expands the workspace section when a new workspace is added (tracks normalWorkspacesList.length), so newly created workspaces are immediately visible even if the user had previously collapsed the section.

Validation

• tsc --noEmit — no errors in changed files
• vitest run NavPanelLayout.test.ts — 4/4 tests pass
• Existing chevron hover behavior (translateX(1px) for scene-link sections) is preserved — the rotation classes only apply to collapsible sections without onSceneOpen

Fixes #976