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

package.json

@@ -3,13 +3,7 @@
   "private": true,
   "type": "module",
   "dependencies": {
-    "@xterm/xterm": "^6.0.0",
-    "@xterm/addon-fit": "^0.10.0",
-    "@xterm/addon-webgl": "^0.18.0",
-    "@xterm/addon-canvas": "^0.7.0",
-    "@xterm/addon-unicode11": "^0.8.0",
-    "@xterm/addon-web-links": "^0.11.0",
-    "@xterm/addon-clipboard": "^0.2.0"
+    "ghostty-web": "^0.1.0"
   },
   "devDependencies": {
     "typescript": "^5.7.0"