feat: add HTML email body and attachment support (#20)

[radiosilence]

Summary

• HTML email body: --html-body (inline) and --html-file (from file) flags on send, reply, and forward. JMAP assembles multipart/alternative automatically when both text and HTML are provided.
• File attachments: -a/--attachment flag (repeatable) on send, reply, and forward. Files uploaded via new upload_blob JMAP method, attached with proper multipart/mixed MIME tree using bodyStructure.
• GraphQL mutations: sendEmail, replyToEmail, forwardEmail accept optional html_body parameter. Previews indicate when HTML is included.
• Refactored body construction: single code path in create_and_submit_email handles plain text, text+HTML, and attachments. Extracted build_compose_params helper in main.rs to deduplicate CLI argument resolution across compose commands.

Closes #20

Test plan

• cargo test — 40 tests pass
• cargo clippy --workspace -- -D warnings — zero warnings
• cargo fmt --check — clean
• Manual: send email with --html-body flag
• Manual: send email with --html-file flag
• Manual: send email with -a attachment.pdf
• Manual: send email with combined HTML + attachments
• Manual: reply/forward with HTML and attachments

🤖 Generated with Claude Code

[radiosilence]

Solid work. Three nits:

1. match ... { Ok => ..., Err(e) => Err(e) } × 3

This pattern in main.rs (lines 488-499, 539-550, 563-574) is a verbose identity on Err. Simplify with ?:

} => {
    let params = build_compose_params(
        cc.as_deref(),
        bcc.as_deref(),
        from.as_deref(),
        draft,
        html_body,
        html_file,
        &attachments,
    )?;
    commands::send(&to, &subject, &body, reply_to.as_deref(), params).await
}

Same for Reply and Forward.

2. upload_blob swallows 4xx errors

The catch-all _ => {} at jmap/mod.rs:980 lets 4xx errors (e.g. 413 payload too large, 400 bad request) fall through to the blobId parse, which fails with a confusing "Upload response missing blobId". Add:

400..=499 => {
    let status = resp.status();
    let text = resp.text().await.unwrap_or_default();
    return Err(Error::Server(format!("Upload failed ({}): {}", status, text)));
}

3. No unit tests for load_attachment / resolve_html

Both are simple but worth covering — especially resolve_html's three branches (inline, file, none) and load_attachment's MIME inference + error on missing file. The test module in util.rs is right there.

None of these are blocking, but worth a quick pass.