fix(paint): backgroundColor now visible with borders and text

Root causes:
- drawBorder wrote plain characters, overwriting fillBackground's ANSI codes
- renderTextWithInlineStyles spread node.props over inherited bg, but
  undefined props from Vue clobbered the inherited backgroundColor
- Child text nodes did not inherit backgroundColor from parent Box

Fixes:
- drawBorder applies backgroundColor (and borderColor) to border chars
- Filter out undefined props before merging in renderTextWithInlineStyles
- Pass inheritedBg through the paint tree to text nodes

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pnpm-lock.yaml

@@ -12,6 +12,9 @@ catalogs:
     '@vitejs/plugin-vue-jsx':
       specifier: ^5.1.5
       version: 5.1.5
+    chalk:
+      specifier: ^5.6.2
+      version: 5.6.2
     vite-plus:
       specifier: latest
       version: 0.1.22
@@ -186,6 +189,9 @@ importers:
       '@vue-tui/testing':
         specifier: workspace:*
         version: link:../testing
+      chalk:
+        specifier: 'catalog:'
+        version: 5.6.2
       typescript:
         specifier: ^6.0.3
         version: 6.0.3