fix: improve Hub download button UX for installed models

[Lokimorty]

Describe Your Changes

When a model is downloaded in Hub, the "Download" button changes to "Use" — but it keeps the same primary button styling. This makes it hard to distinguish installed models at a glance, and "Use" doesn't clearly communicate what happens when you click it.

• Renamed button text from "Use" to "New chat" (with proper translations for all 12 locales)
• Changed button style to gray secondary pattern (matching Settings page buttons like "Check for Updates", "Change Location")

Implementation:

Uses the existing secondary button pattern from Settings:

<Button variant="link" size="sm" className="p-0">
  <div className="rounded-sm hover:bg-main-view-fg/15 bg-main-view-fg/10 ...">
    {t('hub:use')}
  </div>
</Button>

Files changed:

• DownloadButton.tsx — main Hub list button
• hub/$modelId.tsx — model detail page button
• ModelDownloadAction.tsx — variant action button
• 12 locale files with proper translations

Test plan:

1. Download a model from Hub
2. Verify button changes to gray "New chat" (or localized equivalent)
3. Confirm hover state shows lighter gray
4. Click "New chat" → should navigate to new thread with model selected

Fixes Issues

Self Checklist

• Added relevant comments, esp in complex areas
• Updated docs (for bug fixes / features)
• Created issues for follow-up changes or refactoring needed