fix(connections): show inline connecting state and clean up on cancel (#1185)

[datlechin]

Summary

• Replaces the empty/half-rendered editor window during connect with a native ContentUnavailableView showing the database type icon (with .symbolEffect(.pulse)), connection name, host:port, an in-line spinner, and a Cancel button.
• Cancels the in-flight ensureConnected task and drops the .connecting session entry when the user closes the editor window mid-connect, so the libpq result is discarded silently.
• Suppresses the post-cancel "Connection Failed" alert and the ghost "Connecting…" window: WelcomeViewModel/ConnectionFormCoordinator check WindowManager.hasOpenWindow(for:) and only present the alert when the editor window is still open.
• Tears down a successful late connection if the user already closed the window, so we don't leak a connected driver.

Closes #1185.

Test plan

• Connect to a reachable host: connecting state shows briefly, then the editor takes over.
• Connect to an unreachable host (e.g. 192.168.79.78:5432): connecting state shows; clicking Cancel or the X closes the window cleanly with no ghost and no alert.
• Wait for the libpq timeout instead: window closes itself and a single "Connection Failed" alert appears on Welcome.
• Save & Connect from the connection form to a bad host: same suppression applies.