Show DESCRIBE FORMATTED output when a table is not an Iceberg table #201

Context

When a table is not an Iceberg table, the Snapshot selection page and the Graph page show a "not an Iceberg table" error. That behavior is correct, but it stops there: the user doesn't learn what the table actually is and has to switch to another tool to keep debugging.

Concept

IceGraph still won't analyze non-Iceberg tables (Hive, etc.), but it should help the user understand what they're looking at instead of only refusing.

• Backend: a new endpoint exposes a table's DESCRIBE FORMATTED output as JSON. It works for any table Spark can resolve, Iceberg or not.
• Frontend: when the "not an Iceberg table" error appears on the Snapshot selection page or the Graph page, the description loads automatically and is shown with the error, as in "IceGraph can't analyze this table, but here is its description."
• icegraph-client: exposes the new endpoint as well.