feat: add ownership and base model filtering, unify asset/dropdown types (#8497)

Add ownership and base model filtering to AssetBrowserModal and
FormDropdown widgets.

## Changes

- **Ownership filter**: Filter by All/My Models/Public Models (uses
`is_immutable` field)
- **Base model filter**: Multi-select filter with Clear Filters button
- **Type unification**: Replace `AssetDropdownItem` with
`FormDropdownItem`
- **Sorting unification**: Extract shared utilities to
`assetSortUtils.ts`
- **UI refactor**: Use `Button` component, Vue 3.5 prop shorthand, i18n
improvements

---------

Co-authored-by: Amp <amp@ampcode.com>

src/locales/en/main.json

@@ -2504,6 +2504,7 @@
     "imported": "Imported",
     "assetCollection": "Asset collection",
     "assets": "Assets",
+    "baseModel": "Base model",
     "baseModels": "Base models",
     "browseAssets": "Browse Assets",
     "checkpoints": "Checkpoints",
@@ -2558,6 +2559,7 @@
     "selectModelType": "Select model type",
     "selectProjects": "Select Projects",
     "sortAZ": "A-Z",
+    "sortDefault": "Default",
     "sortBy": "Sort by",
     "sortingType": "Sorting Type",
     "sortPopular": "Popular",