Summary

• Adds a Ghostty-style dim overlay on inactive split panes so the active surface stands out at a glance. The mechanism mirrors unfocused-split-fill/unfocused-split-opacity from Ghostty.app: a translucent black tint laid on top of the terminal view (kept below progress/search/drag-handle overlays so those stay legible).
• Tint strength adapts to color scheme — 0.30 in dark mode, 0.12 in light mode — to avoid washing light-theme panes in grey while still giving dark mode the same depth as Ghostty's default.
• New Settings → Appearance → Splits → "Dim unfocused split panes" toggle, defaulting to on. Existing users who prefer the all-bright look can disable it; the value persists through GlobalSettings like every other appearance preference.
• Softens the split divider from .secondary to NSColor.separatorColor so the seam between panes feels neutral instead of busy alongside the new dim treatment.
• Wires focusedSurfaceID through TerminalSplitTreeView / SubtreeView / LeafView (and TerminalSplitTreeAXContainer), with all three call sites — WorktreeTerminalTabsView, ShelfOpenBookView, CanvasCardView/CanvasView — passing state.focusedSurfaceId(in:).

Test plan

• make check
• make build-app
• Open a worktree, create a horizontal split, then a vertical one — confirm only the focused pane stays bright; click between panes to verify the active one swaps cleanly with a short fade.
• Toggle Settings → Appearance → Splits → "Dim unfocused split panes" off, confirm all panes render at full brightness; toggle back on, confirm dim returns.
• Switch the app between light and dark appearance and confirm the dim tint reads as faded (not washed grey) in both.
• Switch to Canvas mode with multiple cards, each containing splits, and confirm dim reflects the focused surface inside each card regardless of which card is selected.
• In a single-pane (no-split) terminal, confirm no dim is applied even when the window is unfocused.
• Verify the split divider line reads as a subtle separator in both light and dark modes.