feat: implement single binary build and env-based auth

- Unify login page styling with share page
- Add AGENTS.md with build commands and code style guidelines
- Add dev.sh and make.sh for development and production builds
- Implement single binary build with embedded frontend using embed.FS
- Change auth configuration from CLI flag to env variables (USHARE_USERNAME, USHARE_PASSWORD)
- Set default credentials: admin / ushare@123
- Fix static file serving for SPA routes

internal/static/static.go

@@ -0,0 +1,14 @@
+package static
+
+import (
+	"embed"
+	"io/fs"
+)
+
+//go:embed frontend/dist
+var FrontendFS embed.FS
+
+func Frontend() fs.FS {
+	sub, _ := fs.Sub(FrontendFS, "frontend/dist")
+	return sub
+}