[feat] Add video help dialog to Upload Model flow (#7177)

## Summary
Adds an interactive video tutorial dialog to help users find CivitAI
model URLs during the Upload Model wizard.

## Changes
- **New Component**: Created reusable `VideoHelpDialog.vue` component
  - Full-width video player with floating close button
  - Configurable props: `videoUrl`, `loop`, `showControls`
  - Custom ESC key handling to prevent parent dialog from closing
  - Click backdrop to dismiss
  - 70% dark backdrop for better video focus
- **Upload Model Flow**: Integrated video help button in step 1 footer
  - "How do I find this?" button opens tutorial video
  - Video demonstrates finding model URLs on CivitAI
- PostHog tracking attribute maintained (`upload-model-step1-help-link`)

## Review Focus
- ESC key event handling uses capture phase to prevent propagation to
parent dialogs
- Component follows existing patterns from `MediaVideoTop.vue` and
`BaseModalLayout.vue`
- Video player accessibility (ARIA labels, keyboard navigation)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7177-feat-Add-video-help-dialog-to-Upload-Model-flow-2c06d73d36508148963ad9ee60038ea3)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Claude <noreply@anthropic.com>

src/locales/en/main.json

@@ -2113,6 +2113,8 @@
     "uploadingModel": "Importing model...",
     "uploadSuccess": "Model imported successfully!",
     "uploadFailed": "Import failed",
+    "uploadModelHelpVideo": "Upload Model Help Video",
+    "uploadModelHowDoIFindThis": "How do I find this?",
     "modelAssociatedWithLink": "The model associated with the link you provided:",
     "modelTypeSelectorLabel": "What type of model is this?",
     "modelTypeSelectorPlaceholder": "Select model type",