Replace textual-serve with direct xterm.js 6.0 bundle

- Add package.json with @xterm/xterm 6.0 and all addons
- Create terminal.ts client with WebSocket protocol support
- Bundle with Bun (bun run build -> terminal.js)
- Remove textual-serve dependency from pyproject.toml
- Remove canvas monkey-patch workaround (no longer needed)
- Add scrollback support (configurable via data-scrollback)
- Update static file routing to serve from /static/
- Add Makefile targets: bundle, bundle-watch, bundle-clean
- Update tests for new static path structure

Benefits:
- Full control over xterm.js configuration
- Scrollback history now works (default 1000 lines)
- Custom font family without workarounds
- Smaller footprint (no unused Roboto Mono fonts)
- Latest xterm.js 6.0 features available

.gitignore

@@ -129,3 +129,10 @@ dmypy.json
 
 # textual-webterm specific
 textual.log
+
+# Node.js / Bun
+node_modules/
+bun.lockb
+
+# Built JS bundle (regenerate with: bun run build)
+src/textual_webterm/static/js/terminal.js