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>

.gitignore

@@ -1,11 +1,22 @@
+# Dependencies
 node_modules/
-dist/
-tmp/
-static/
-*.db
-*.exe
-download/
+
+# Build output
 build/
-.VSCodeCounter/
-.claude/
-server/bilidown.exe~
+
+# IDE
+.idea/
+.vscode/
+*.swp
+*.swo
+
+# OS
+.DS_Store
+Thumbs.db
+
+# Project specific
+.omc/
+backup/
+*.db
+download/
+bin/