Add navigable world model previews

[endfish]

Summary

• Adds a reusable world-model preview workflow to the catalog.
• Adds Previous and Next navigation plus direct numeric index jumps.
• Synchronizes metadata selection, list scrolling, and the currently spawned preview entity.
• Adds a focused model-preview window with navigation and an explicit Apply This Model action.
• Adds direct access to world-model preview browsing from Brio's spawn menu.

Motivation

World-object entries are often identified primarily by internal paths and codes. Spawning each entry permanently just to discover what it represents is slow and leaves extra entities to clean up. A persistent preview entity makes browsing the catalog much faster: selecting or navigating to another entry replaces the existing preview in place.

Implementation notes

• Reuses a single Brio-managed preview entity instead of accumulating spawned objects.
• Cancels or disposes stale asynchronous preview results when the selection changes quickly.
• Cleans up previews when the preview window or catalog closes and when GPose ends.
• Keeps the metadata list selection and scroll position synchronized with Previous/Next and numeric navigation.
• Treats a missing user metadata store as empty on a new installation instead of reporting it as a corrupt file.
• Fixes IconButtonWithText layout so subsequent same-line controls use the full button bounds.
• Contains English UI text only; no Chinese resources or Brio-CN-specific behavior are included.

Validation

• dotnet build brio.slnx -c Debug
• Result: 0 warnings, 0 errors