Asset browser: implement optional lateral folder tree panel

Goal

Implement an optional lateral panel in the asset browser showing the complete folder structure as a collapsible tree widget, enabling efficient navigation without the current folder-traversal button row. When this panel is active, hide the .. "go up" button since tree-based navigation makes it redundant.

Context / integration points

  • Asset browser currently displays a breadcrumb + folder buttons (.., refresh); the breadcrumb is static.
  • The folder tree panel should coexist with the current grid view (or switchable).
  • Related to #68 (closed) (clickable breadcrumb) and #70 (closed) (tree-exclusive navigation) — this is the foundation.
  • Shares UI patterns with scene hierarchy panel (both are hierarchical tree widgets over filesystem).

Design

  • Add a toggle in the header to show/hide the lateral tree panel (similar to "Details" panels in many editors).
  • Tree widget displays the asset/ directory structure recursively; can be collapsed/expanded per folder.
  • Clicking a folder in the tree navigates the main grid view to that folder.
  • When tree panel is visible, hide the .. button in the main grid header (breadcrumb navigation via tree).

Tasks

  • Implement collapsible tree widget component (reusable, not specific to asset browser)
  • Wire tree widget to scan the asset/ directory and build the hierarchy
  • Add toggle button in asset browser header to show/hide the tree panel
  • Navigate main grid view when tree node is clicked
  • Hide .. button when tree panel is visible
  • Persist toggle state in settings (remember if tree was open/closed last session)

Acceptance criteria

  • Tree panel displays the full asset/ hierarchy with all subdirectories.
  • Clicking a folder in the tree updates the main grid to show that folder's contents.
  • Tree can be toggled on/off; .. button hides when tree is visible.
  • Navigating via tree is as fast as the current button-based approach.

Dependencies

Related to #68 (closed) (breadcrumb), #70 (closed) (tree-exclusive mode). Unblocked; can ship independently.

Out of scope

  • File display in the tree (that's #70 (closed), tree-exclusive mode).
  • Drag-drop reorganization of assets.
  • Advanced tree filtering/search (future enhancement).