refactor: move CLI to clap derive (no behavior change) + prep modular CLI

[Mathious6]

Why?

The builder API in main.rs was getting long and hard to understand, with derive + typed fields (with validators) we will reduce runtime parsing bugs. Regarding tests, a single Cli struct is easy to unit test.

What?

• New: src/cli.rs defines the full CLI using clap’s derive API.
• Validation at parse time:
  • account / from / to now validated as 32 hex chars (parse_account_id).
  • quantity validated as >= 1.
• Documentation cleanup: minor help text polish, clearer examples.

Note

I considered introducing an AccountId newtype (with FromStr) now, but left it for a follow-up to keep this PR focused and small.

Main impact?

• End users: no behavior change; --help is cleaner. Old flags still work via visible aliases.
• Developers: CLI shape is defined in one place (cli.rs). The dispatcher in lib.rs keeps working because argument IDs are unchanged.
• Safety: invalid account IDs and zero quantities are rejected earlier with clear error messages.

What next?

• Introduce AccountId (in bourso_api) to solve primitive obsession properly. Implement FromStr and use it in CLI + API.

• Split handlers into modules (small, testable functions):

  src/
  ├─ main.rs           # init + parse + call run()
  ├─ lib.rs            # run() dispatcher
  ├─ cli.rs            # clap structs (this PR)
  ├─ commands/         # accounts.rs, quote.rs, trade/, transfer.rs
  ├─ services/         # client.rs, auth.rs (MFA/login)
  └─ settings/         # as today
  

  Example target:

  pub async fn run(cli: cli::Cli) -> Result<()> {
      use cli::Commands::*;
      match cli.command {
          Config(args)   => commands::config::handle(args).await,
          Quote(args)    => commands::quote::handle(args).await,
          // ...
      }
  }

[azerpas]

There was a reason for which I didn't used derive at first, but I can't put my head around it... Anyway, awesome refactoring job once again, thanks Mathis :)