Show skeleton loading in data/structure view instead of small spinner

[datlechin]

Two UX improvements for loading states:

1. Skeleton loading for data/structure views: When a table is first loading (no records yet), show a skeleton placeholder in the data grid area instead of the tiny centered spinner. On slow connections the current spinner is easy to miss, making the app look broken.

2. Clear stale data when switching tables: When clicking a different table in the sidebar, immediately clear the previous table's data and show the skeleton loading state. Right now the old data stays visible until the new table loads, which feels like "dirty data" since it's no longer relevant.