fix(terminal): preserve live sessions when moving tabs between groups

[htazq]

Summary

• keep live tab content mounted in stable per-tab portal containers
• let the recursive grid move lightweight portal hosts instead of remounting terminals
• preserve active-group behavior, active-tab visibility, reconnect behavior, and normal cleanup when a tab is actually closed
• add a regression test that moves a live terminal tab into a new group without mounting or unmounting either terminal again

Root cause

MOVE_TAB_TO_NEW_GROUP replaces a grid leaf with a branch. React consequently unmounts the old TerminalGroupView, which previously owned every live PtyTerminal in the source group. The cleanup closes each WebSocket and backend PTY, and the replacement layout starts new shells.

Stable keys cannot preserve components that move across different parents or whose parent hierarchy changes, so this separates live tab ownership from the layout tree.

User impact

Moving or splitting tabs now changes only their layout. Existing xterm instances, WebSocket connections, PTY sessions, terminal history, and running foreground commands stay alive.

Testing

• pnpm test -- src/__tests__/terminal-tab-portals.test.tsx src/__tests__/grid-renderer.test.tsx ? 2 files, 5 tests passed
• pnpm build ? passed
• scoped ESLint for the three changed terminal components ? passed with one pre-existing grid-renderer.tsx warning
• full Vitest run ? 31 files / 467 tests passed; 14 pre-existing environment/mock failures remain in connection.test.ts, pty-terminal-activation.test.tsx, and pty-terminal-scrollbar.test.tsx
• full ESLint remains blocked by the pre-existing settings-modal.tsx:293 error

Fixes #46
Related to #7

[GOODBOY008]

@htazq LGTM