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

Makefile

@@ -1,10 +1,11 @@
-.PHONY: help install install-dev lint format test coverage check clean
+.PHONY: help install install-dev lint format test coverage check clean bundle bundle-watch bundle-clean
 
 PYTHON ?= python3
 PIP ?= $(PYTHON) -m pip
 
 help:
 	@echo "Targets: install install-dev lint format test coverage check clean"
+	@echo "Frontend: bundle bundle-watch bundle-clean"
 
 install:
 	$(PIP) install -e .
@@ -29,3 +30,16 @@ check: lint coverage
 
 clean:
 	rm -rf .pytest_cache .coverage htmlcov .ruff_cache
+
+# Frontend build targets (requires Bun: https://bun.sh)
+node_modules: package.json
+	bun install
+
+bundle: node_modules
+	bun run build
+
+bundle-watch: node_modules
+	bun run watch
+
+bundle-clean:
+	rm -rf node_modules bun.lockb src/textual_webterm/static/js/terminal.js