feat(tui): uniform selection background instead of SGR inverse

Selection was falling back to SGR-7 inverse (fg ↔ bg per cell), which
fragments over syntax-highlighted content — each amber/gold/dim/cornsilk
fg turned into a different bg stripe, producing the staircase look.

Now `useMainApp` calls `selection.setSelectionBgColor()` with a muted
navy (`#3a3a55`) on theme change. `setSelectionBg` in screen.ts replaces
just the bg cell-by-cell while preserving fg/bold/dim/italic, so the
highlight is one solid color across the whole drag range and the text
stays readable in its original color.

Skins can override via `selection_bg` in their color map.

ui-tui/src/app/useMainApp.ts

@@ -130,6 +130,13 @@ export function useMainApp(gw: GatewayClient) {
   const hasSelection = useHasSelection()
   const selection = useSelection()
 
+  // Bind a uniform selection bg so drag-to-select shows one solid color
+  // across the whole range instead of SGR-inverse (which swaps each cell's
+  // fg → bg and fragments over amber/gold/dim text). Re-fires on skin swap.
+  useEffect(() => {
+    selection.setSelectionBgColor(ui.theme.color.selectionBg)
+  }, [selection, ui.theme.color.selectionBg])
+
   const composer = useComposerState({
     gw,
     onClipboardPaste: quiet => clipboardPasteRef.current(quiet),