(grid): refit follow-ups (group toggle double fit, WebGL lookup by id) #864

Follow-up to #545 / #860 (round-b review approved; two minors left).

1. "Group by project" toggle fits visible cards twice (public/grid-view.js ~374-377). setGridGroupByProject still calls fitAndScroll on every session, then each new card's first observer callback runs refitVisibleGridTerminal again (measured: fits 2 instead of 1); off-screen cards are also fitted. Replace the loop with refitOpenTerminals() like showGridView, and add the toggle to the route test loop in test/terminal-grid-refit.test.js.
2. restoreTerminalWebgl(entry.session.sessionId) re-looks the entry up by id (public/terminal-manager.js ~319) although the frame already holds it; a re-key between steps could load WebGL on the wrong entry. Call loadTerminalWebgl(entry) directly.