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/integration/components/background-color.test.tsx

@@ -0,0 +1,47 @@
+import chalk from "chalk";
+import { afterEach, beforeEach, expect, test } from "vite-plus/test";
+import { render } from "@vue-tui/testing";
+import { Box, Text } from "@vue-tui/runtime";
+
+const BG_BLUE = "\x1b[44m";
+
+let prevLevel: typeof chalk.level;
+beforeEach(() => {
+  prevLevel = chalk.level;
+  chalk.level = 1;
+});
+afterEach(() => {
+  chalk.level = prevLevel;
+});
+
+test("Box backgroundColor produces ANSI background codes", async () => {
+  const { frames } = await render(
+    () => <Box backgroundColor="blue" width={5} height={1} />,
+    { columns: 10 },
+  );
+  expect(frames.at(-1)).toContain(BG_BLUE);
+});
+
+test("Box backgroundColor survives border rendering", async () => {
+  const { frames } = await render(
+    () => <Box backgroundColor="blue" borderStyle="single" width={6} height={3} />,
+    { columns: 10 },
+  );
+  const raw = frames.at(-1)!;
+  expect(raw).toContain(BG_BLUE);
+  expect(raw).toContain("┌");
+});
+
+test("child Text inherits backgroundColor from parent Box", async () => {
+  const { frames } = await render(
+    () => (
+      <Box backgroundColor="blue" width={10} height={1}>
+        <Text>hello</Text>
+      </Box>
+    ),
+    { columns: 20 },
+  );
+  const raw = frames.at(-1)!;
+  expect(raw).toContain("hello");
+  expect(raw).toContain(BG_BLUE);
+});

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"