Fix active pane border and pane opacity dimming showing simultaneously on editor panes and terminal panes

[usizu]

When active_pane_modifiers.border_size is set in settings.json, both the active editor pane and the active terminal pane would display a border simultaneously, making it impossible to tell which pane actually has focus. The pane opacity dimming also renders simultaneously before this fix.

1. Before fix: impossible to tell which pane has focus

2. After fix: only one pane has focus

The root cause is that both the workspace center and the terminal panel pass their own active_pane to PaneRenderContext, so each pane group unconditionally marks its active pane for border rendering. The fix adds a focus check so active_pane_ix is only set when the pane actually has keyboard focus, ensuring only the truly focused pane gets the border (and the inactive_opacity dimming applies correctly to unfocused groups).

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Release Notes:

• Fixed active pane border being drawn on both the editor pane and terminal pane simultaneously