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/theme.ts

@@ -19,6 +19,8 @@ export interface ThemeColors {
   statusBad: string
   statusCritical: string
 
+  selectionBg: string
+
   diffAdded: string
   diffRemoved: string
   diffAddedWord: string
@@ -94,6 +96,11 @@ export const DEFAULT_THEME: Theme = {
     statusBad: '#FF8C00',
     statusCritical: '#FF6B6B',
 
+    // Uniform selection bg — matches the muted navy of the status bar so
+    // gold/amber fg stays readable and the highlight doesn't fragment per
+    // fg color the way SGR-inverse does.
+    selectionBg: '#3a3a55',
+
     diffAdded: 'rgb(220,255,220)',
     diffRemoved: 'rgb(255,220,220)',
     diffAddedWord: 'rgb(36,138,61)',
@@ -149,6 +156,8 @@ export function fromSkin(
       statusBad: d.color.statusBad,
       statusCritical: d.color.statusCritical,
 
+      selectionBg: c('selection_bg') ?? d.color.selectionBg,
+
       diffAdded: d.color.diffAdded,
       diffRemoved: d.color.diffRemoved,
       diffAddedWord: d.color.diffAddedWord,