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

.gitignore

@@ -1,5 +1,19 @@
+# IDE
 .idea
 .vscode
+
+# OS
 .DS_Store
+
+# Build output
 dist
-xtest
+
+# Data directories
+data/
+x-*/
+
+# Temporary build files
+internal/static/frontend
+
+# Compiled binaries
+ushare