fix: propagate keep-running policy changes to background panes

[jasonsuhari]

Problem

Changing Keep terminals running updates visible panes and panes in inactive tabs, but not background jobs. Background panes therefore retain an outdated persistence policy and can survive or terminate contrary to the saved setting.

Acceptance checks

• Live background panes receive policy updates.
• Offline background entries remain unchanged.
• The first propagation error is surfaced without skipping other panes.
• Regression coverage includes all pane containers.