TerminalPanel was connecting on mount even when the drawer was closed
and the terminal tab was inactive. Combined with reconnectAttempts
resetting on every ws.onopen, this caused infinite reconnection loops
that spawned PTY processes until system limits were hit.
- Pass `visible` prop to TerminalPanel, only connect when terminal tab
is actually shown
- Move reconnectAttempts reset from ws.onopen to "created" handler so
only successful PTY creation resets the counter
- Remove unused onMounted import
Fixes#509
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>