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/components/UploadModelUrlInputCivitai.vue

@@ -38,13 +38,19 @@
           }}</span>
         </template>
       </i18n-t>
-      <InputText
-        v-model="url"
-        autofocus
-        :placeholder="$t('assetBrowser.civitaiLinkPlaceholder')"
-        class="w-full bg-secondary-background border-0 p-4"
-        data-attr="upload-model-step1-url-input"
-      />
+      <div class="relative">
+        <InputText
+          v-model="url"
+          autofocus
+          :placeholder="$t('assetBrowser.civitaiLinkPlaceholder')"
+          class="w-full border-0 bg-secondary-background p-4 pr-10"
+          data-attr="upload-model-step1-url-input"
+        />
+        <i
+          v-if="isValidUrl"
+          class="icon-[lucide--circle-check-big] absolute top-1/2 right-3 size-5 -translate-y-1/2 text-green-500"
+        />
+      </div>
       <p v-if="error" class="text-xs text-error">
         {{ error }}
       </p>
@@ -73,8 +79,11 @@
 
 <script setup lang="ts">
 import InputText from 'primevue/inputtext'
+import { computed } from 'vue'
 
 import { useFeatureFlags } from '@/composables/useFeatureFlags'
+import { civitaiImportSource } from '@/platform/assets/importSources/civitaiImportSource'
+import { validateSourceUrl } from '@/platform/assets/utils/importSourceUtil'
 
 const { flags } = useFeatureFlags()
 
@@ -83,4 +92,10 @@ defineProps<{
 }>()
 
 const url = defineModel<string>({ required: true })
+
+const isValidUrl = computed(() => {
+  const trimmedUrl = url.value.trim()
+  if (!trimmedUrl) return false
+  return validateSourceUrl(trimmedUrl, civitaiImportSource)
+})
 </script>