[feat] Add PostHog data-attr attributes to Upload Model flow (#7173)

## Summary
Adds step-specific `data-attr` attributes throughout the Upload Model
wizard to enable PostHog analytics tracking and action creation,
following PostHog's recommended best practices for element labeling.

## Changes
- **What**: Added `data-attr` attributes to all key interactive elements
in the Upload Model flow (buttons, inputs, selectors)
- **Step-based naming**: Attributes include step numbers for funnel
analysis (e.g., `upload-model-step1-continue-button`)
- **Coverage**: Entry button, URL input, help link, navigation buttons
(cancel/back/continue/confirm/finish), and model type selector

## Benefits
- Enables creation of stable PostHog actions using CSS selectors like
`[data-attr="upload-model-step2-confirm-button"]`
- Allows funnel analysis to track user progression through the 3-step
upload wizard
- Identifies drop-off points and help-seeking behavior
- Follows PostHog best practice of using data attributes over CSS
classes (especially important with Tailwind)

## Review Focus
- Naming consistency and clarity of data-attr values
- Completeness of coverage across the upload flow

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

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7173-feat-Add-PostHog-data-attr-attributes-to-Upload-Model-flow-2c06d73d365081699861d3d910250e32)
by [Unito](https://www.unito.io)

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

src/platform/assets/components/AssetBrowserModal.vue

@@ -34,6 +34,7 @@
           type="accent"
           size="md"
           class="!h-10 [&>span]:hidden md:[&>span]:inline"
+          data-attr="upload-model-button"
           :label="$t('assetBrowser.uploadModel')"
           :on-click="showUploadDialog"
         >