fix(terminal): preserve fullscreen under modals

[matej21]

Summary

• Keep the fullscreen terminal presentation active while a modal owns keyboard focus.
• Derive the effective fullscreen state through modal focus-stack entries.
• Add a regression test for opening and closing a modal over a fullscreen terminal.

Root cause

Fullscreen queries required the current focus context to be exactly Fullscreen. The Send to Terminal composer enters the Modal context so its input keeps keyboard focus. That made the renderer fall back to the focused parent-project view and reveal its worktrees beside the terminal.

Impact

Opening the composer, search, or another modal over a fullscreen terminal no longer changes the underlying layout. Modal keyboard focus and terminal focus restoration remain unchanged.

Validation

• cargo test -p okena-workspace — 374 passed