Improve global.css as scaffolding template (#403)

Update `global.css` in all `basic-server-*` examples (and `integration-server`
for consistency) to better serve as templates for AI coding agents scaffolding
new MCP Apps.

Changes:
- Use CSS custom properties (`--font-sans`, `--color-text-primary`, etc.) with
  sensible fallbacks, enabling host design system integration
- Add `color-scheme: light dark` and `light-dark()` fallbacks for theme support
- Include comprehensive typography: h1-h6, body text, `code`/`pre`/`kbd`, `b`/`strong`
- Move `#server-time` styles from global CSS to component stylesheets (classes
  instead of IDs for better CSS practices)

Updates E2E test selectors in `DYNAMIC_MASKS` to match the new class-based
selectors.

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

examples/basic-server-react/src/mcp-app.module.css

@@ -70,3 +70,9 @@
     font-style: normal;
   }
 }
+
+/* Server time fills remaining width for consistent E2E screenshot masking */
+.serverTime {
+  flex: 1;
+  min-width: 0;
+}