refactor: refactor for modern layout

- split frontend and backend.
- update backend with modern Python dev strategies.

backend/.gitignore

@@ -0,0 +1,19 @@
+## ======== Personal ========
+# Database file
+*.db
+
+# Ignore setting file
+coconut-leaf.toml
+
+## ======== Python ========
+# Python-generated files
+__pycache__/
+*.py[oc]
+build/
+dist/
+wheels/
+*.egg-info
+
+# Virtual environments
+.venv
+