fix(DataTable): make default row hover visible on dark theme

[sunlight7777777]

Summary

Fixes the invisible row hover state on DataTable. The default used
surface.subtle, which on our dark theme is indistinguishable from
the table background.

Closes #1278

Change

Single-file CSS change in src/components/common/DataTable.tsx:

• Hover background: surface.subtle → border.subtle
• Added borderBottom: 1px solid surface.light for row separation
• Widened transition from background-color to all

This matches the styling TopRepositoriesTable already uses via its
own getRowSx override.

Affected surfaces

• ✅ RepositoryIssuesTable — was broken, now visibly hoverable
• ✅ Any other DataTable consumer relying on the default — same fix
• ⚪ TopRepositoriesTable — no visual change (its consumer-level
  override matches the new default)

Testing

• Repository → Issues tab: rows highlight on hover, cursor changes
  to pointer, click opens GitHub.
• Repositories list view: hover behavior unchanged.
• Lint passes.
• Type check passes.

##Screenshot
Before Change:

bandicam.2026-05-26.18-34-33-697.mp4

After Change:

bandicam.2026-05-26.18-35-24-742.mp4

[sunlight7777777]

My prs are merged so have only one open pr now but #1287 pr is still closed
I should create new pr for #1258 ?
@anderdc
@ventura-oss