Replace xterm.js with ghostty-web

Migrate from xterm.js to ghostty-web (Ghostty WASM terminal emulator).

Benefits:
- WASM-compiled parser from Ghostty (same code as native app)
- Better Unicode/complex script handling
- Simpler initialization (no viewport.scrollBarWidth issues)
- ~400KB WASM bundle

Changes:
- Update package.json: remove @xterm/* deps, add ghostty-web
- Rewrite terminal.ts using ghostty-web API
- Use built-in FitAddon with observeResize()
- Remove WebGL/Canvas/Unicode11/WebLinks/Clipboard addons
- Remove xterm.css (ghostty uses canvas renderer)
- Add ghostty-vt.wasm to static assets
- Update HTML template and tests

BREAKING: Major version bump to 0.4.0

tests/test_local_server.py

@@ -18,7 +18,7 @@ class TestLocalServer:
         """Test that static path contains required assets."""
         assert WEBTERM_STATIC_PATH is not None
         assert (WEBTERM_STATIC_PATH / "js" / "terminal.js").exists()
-        assert (WEBTERM_STATIC_PATH / "css" / "xterm.css").exists()
+        assert (WEBTERM_STATIC_PATH / "js" / "ghostty-vt.wasm").exists()
 
     def test_create_server(self, tmp_path) -> None:
         """Test creating a LocalServer instance."""