feat(ui): add search filter to provider grid

[harrykamboj1]

Summary

Adds a search input to the Providers grid so users can quickly filter
providers by name or description instead of scanning the full card grid.

• New search input with a Search icon above the grid
  (data-testid="provider-search-input")
• Case-insensitive filtering on provider display_name and description
  (query is trimmed)
• "No providers match "{query}"" empty-state message when a search yields
  no results
• New i18n messages: providerGrid.searchPlaceholder, providerGrid.noMatch
• Moved the GridLayout wrapper from ProviderGrid into ProviderCards
  so the search bar and empty-state text render outside the grid

Applies to both the onboarding provider selection flow and the settings
page, since both render ProviderGrid.

Testing

• cd ui/desktop && pnpm run typecheck — passes
• cd ui/desktop && pnpm test — passes
• Manual testing via just run-ui → Settings → Providers:
  • Empty query renders the full sorted grid, unchanged from today
  • Typing filters cards live by name and description (case-insensitive)
  • A query with no matches shows the empty-state message
  • Clearing the query restores the full grid
  • Verified in both light and dark mode

Related Issues

Relates to #10436
Discussion: N/A

Screenshots/Demos (for UX changes)