fix(assets): preserve companion file paths and migrate existing flat textures

[JeanPhilippeKernel]

Problem

The Poly Haven API returns companion file relative paths as inc.Key (e.g. textures/foo_diff_4k.jpg, foo.bin). The previous code only stored the URL and derived the filename via Path.GetFileName, silently dropping the subdirectory prefix. All textures were placed flat next to the primary model file, breaking texture references in GLTF, FBX, Blend, and USD files.

Changes

Fix: new downloads use correct structure

• AssetFormatOption: CompanionUrls List<string> → CompanionFiles List<(RelativePath, Url)>
• LoadAssetFormatsAsync: store (inc.Key, url) instead of url only
• LoadAndDownloadAssetAsync: use RelativePath verbatim relative to destDir, creating subdirectories (textures/) as needed

Fix: background migration for existing assets

• MigrateAssetFoldersAsync runs once at startup in the background
• Finds all flat image files (jpg/jpeg/png/webp/hdr/exr) under AppPaths.Assets
• Moves them into textures/ alongside their primary model file
• Shows a toast before migration begins and after completion
• Idempotent — skips files that are already in the correct location
• Non-fatal — any individual move failure leaves the file in place

Result

assets/{assetId}/{resolution}/
├── {model}.gltf / .fbx / .blend / .usd   ← texture refs now resolve
├── {model}.bin                            ← GLTF buffer stays flat
└── textures/
    ├── {model}_diff_{res}.jpg
    ├── {model}_nor_gl_{res}.jpg
    └── ...

Verified against GLTF (wine_barrel_01, chandelier_03), FBX, Blend — all formats use textures/ consistently per the Poly Haven API.