fix(connections): keep one connection per database, no reconnect

[datlechin]

Why

Clicking between two database entries of one PostgreSQL connection reconnected every time: moveSessionOntoDatabase tore the session down and opened a new one. The database left lost its open transaction, temp tables and SET values silently, so a later COMMIT reported success for rolled-back writes. PostgreSQL cannot USE another database, but it can hold a second connection.

Change

• Each database browsed from the strip keeps its own session connection (SessionLanes). Switching parks the one left and swaps in the target, with no reconnect. Closing the entry, a rename or drop, a rebuilt tunnel or a disconnect closes it.
• A tab runs on its database's connection, not a pooled metadata one the Structure tab shared and the pool closed when idle.
• A parked connection that died in a transaction is replaced; the next statement is refused once.
• Import and Copy To join a transaction the user holds instead of committing it. PostgreSQL connections send TCP keepalives, and a ping no longer reconnects a session in a failed transaction.

Verified

SessionLanesTests covers switch, reuse, refusal, teardown and transaction loss. Live on PostgreSQL 17 with log_connections=on: three clicks between two entries opened and closed nothing.

Not in this PR

• The database's catalog still reloads on each switch; keeping one per database is a follow-up.
• No confirmation before closing an entry or disconnecting with an open transaction.
• The website's PostgreSQL limit "Changing database means reconnecting" needs rewording at release.

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