unfocused-split-opacity and unfocused-split-fill config settings not respected

[Whoaa512]

Description

Ghostty's unfocused-split-opacity and unfocused-split-fill config settings have no effect in Supacode.

Upstream Ghostty reads these values via Ghostty.Config and applies a semi-transparent overlay on unfocused split panes in SurfaceWrapper. Supacode uses its own TerminalSplitTreeView which doesn't implement this overlay.

Steps to reproduce

1. Add unfocused-split-opacity = 0.5 to ~/.config/ghostty/config
2. Open a worktree in Supacode
3. Create a split (right-click → Split Right)
4. Click between splits

Expected

The unfocused split pane should be visually dimmed per the config value.

Actual

Both panes look identical regardless of focus state.

Suggested fix

• Read unfocused-split-opacity and unfocused-split-fill via ghostty_config_get from GhosttyRuntime
• Track surface focus state on GhosttySurfaceState (already @Observable)
• Apply a Rectangle overlay in TerminalSplitTreeView.LeafView when the surface is in a split and not focused