phsl / new-api

The header’s skeleton screen now remains visible for at least 500 ms and
only disappears after `/api/status` has successfully populated
`StatusContext`.

Changes include:
• Added `loadingStartRef` to record the mount time.
• Reworked loading effect to compute the remaining delay based on the
  elapsed time and the presence of real status data.
• Removed the previous fixed‐timer logic, preventing premature content
  rendering and improving perceived loading consistency across pages.