refactor: restructure project to standard Go layout

- Reorganize code into cmd/bilidown and internal/ packages
- Rename client/ to web/ for frontend source
- Remove systray dependency for headless web service
- Embed static files into binary using go:embed
- Update import paths to use internal/ prefix
- Update .gitignore with common patterns

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/tsconfig.json

@@ -0,0 +1,8 @@
+{
+    "compilerOptions": {
+        "strict": true,
+        "module": "ESNext",
+        "moduleResolution": "Bundler",
+        "target": "ESNext"
+    }
+}