Migrate thread terminals to grouped layout model

- Replace `terminalLayout`/`splitTerminalIds` with `terminalGroups` and `activeTerminalGroupId`
- Update terminal drawer UI to render grouped splits with a terminal sidebar
- Bump persisted web state to v7 with legacy v6 migration coverage in tests
a435fa5a21Julius Marminge committed on 2/13/2026, 5:06:03 AMparent817c5e6
9 files changedLine totals unavailable