refactor: product create and edit share their field flags

[alukach]

Stacked on #47.

product create and product edit declared -t/--title, -d/--description, --visibility and -F/--from-file identically. They now live in one Fields struct in src/product.rs, flattened into CreateArgs and EditArgs. The request body, prompting order and everything else are unchanged.

Behaviour unchanged. I diffed --help for both commands before and after. Flags, short options and value names are the same. Only two things differ:

• --title and --description now have help text ("The product's title" / "The product's description").
• Options are in a different order: --data-connection (create) and --disable/--enable (edit) now come after --from-file.

Checks: cargo fmt, cargo clippy --all-targets -- -D warnings, cargo test and cargo test --features staging all pass, including resource_commands_follow_the_cli_conventions.

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