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>

internal/common/common.go

@@ -0,0 +1,14 @@
+package common
+
+import (
+	"crypto/rand"
+	"fmt"
+)
+
+func RandomString(length int) string {
+	randomBytes := make([]byte, length)
+	rand.Read(randomBytes)
+	return fmt.Sprintf("%x", randomBytes)[:length]
+}
+
+type MediaFormat int