fix: Model upload UI improvements (#7938)

## Summary

Polishing improvements for the model upload (BYOM) experience.

## Changes

- **HoneyToast z-index**: Increased from `z-50` to `z-9999` so the
ModelImportProgressDialog appears above modal backdrops
- **VideoHelpDialog**: Removed pixel-based max-width constraint, now
uses `90vw` to fill more of the viewport
- **UploadModelDialog responsive layout**: Added `max-height: 90vh` and
scrollable content area to prevent footer buttons from underflowing on
small screens
- **URL validity indicator**: Added green checkmark icon inside the URL
input when a valid Civitai or HuggingFace URL is entered

## Testing

- Open the model upload dialog and verify buttons remain accessible on
small viewport heights
- Enter a valid Civitai/HuggingFace URL and confirm the green checkmark
appears
- Open the help video and verify it uses more of the viewport
- Start a model download and verify the progress toast appears above any
open modals

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7938-fix-Model-upload-UI-improvements-2e46d73d365081a292f5fda70c6db0f5)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Amp <amp@ampcode.com>
Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

src/platform/assets/composables/useModelUpload.ts

@@ -23,7 +23,7 @@ export function useModelUpload(
         dialogComponentProps: {
           pt: {
             header: 'py-0! pl-0!',
-            content: 'p-0!'
+            content: 'p-0! overflow-y-hidden!'
           }
         }
       })
@@ -41,7 +41,7 @@ export function useModelUpload(
         dialogComponentProps: {
           pt: {
             header: 'py-0! pl-0!',
-            content: 'p-0!'
+            content: 'p-0! overflow-y-hidden!'
           }
         }
       })