Asset browser: tree-exclusive navigation mode (REPLACEMENT)

Goal

Allow the asset browser to operate in an exclusive tree-only navigation mode (like Unity), where the folder structure tree contains both folders and files, and all operations (preview, selection, context menu) flow through the tree. The main grid view becomes optional or disappears entirely in this mode.

Context / integration points

Related to #67 (closed) (tree panel) — this is the "full tree" variant.
Requires the tree widget to display files (not just folders) with icons/previews inline or on-hover.
Must integrate with the preview API (#19 (closed)) to show asset previews on tree selection.
Some users prefer tree-only workflows; this is an optional mode.

Design

Add a "Navigation Mode" toggle/setting: Grid + Tree (default) vs. Tree Only.
In Tree Only mode:
- Tree fills the left side (or entire panel).
- Tree nodes include both folders (expandable) and files (leaf nodes).
- Clicking a file shows its preview on the right (or inline).
- Right-click on files/folders opens context menus (create, delete, etc.).
The tree widget becomes the primary selection interface.

Tasks

Extend tree widget to render files as leaf nodes (not just folders)
Add file icons/thumbnails to tree nodes
Implement file selection and preview on tree click
Add context menu support for tree files/folders
Add a toggle/setting to switch between Grid+Tree and Tree-Only modes
Integrate with preview API (#19 (closed)) for inline previews

Acceptance criteria

Tree-Only mode displays all files and folders in the tree.
Selecting a file in the tree shows its preview (if supported).
Context menu works on tree nodes (create, delete, rename, etc.).
Toggle between modes preserves the current folder/selection.
No performance regression when tree contains many files.

Dependencies

Depends on #67 (closed) (tree panel foundation) and #19 (closed) (preview API). Can coexist with grid view.

Out of scope

Drag-drop reorganization in tree.
Inline renaming of files in the tree.