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>

packages/runtime-tests/package.json

@@ -12,6 +12,7 @@
     "@vitejs/plugin-vue-jsx": "catalog:",
     "@vue-tui/runtime": "workspace:*",
     "@vue-tui/testing": "workspace:*",
+    "chalk": "catalog:",
     "typescript": "^6.0.3",
     "vite-plus": "^0.1.20",
     "vue": "^3.4.0"