Refactor: Move all @tauri-apps calls to src/commands

[maidh91]

Context

• This is one step in the plan to support this app running on web browser, and mobile
• Goal
  • in Desktop mode, src/commands will calls to @tauri-apps
  • in Web browser mode, src/commands will calls to some backend
    • eventually, the dir structure can be like this

wealthfolio/
├── src/
├── src-tauri/
├── src-rust-api/    # some REST api backend
├── src-rust-lib/    # shared rust code using `src-tauri` and `src-rust-api` 

Summary

• Move all @tauri-apps calls to src/commands

  • This src/commands directory contains all actions to communicate with Tauri via @tauti-apps library, e.g. invoke, listen, open
  • All other directories under src are native React code only

• Add README.md to src/commands to note the convention