feat: add HTML email body for --send (#48)

Add email.html config flag (default false). When enabled, --send
includes an HTML version of the report body generated via HTMLFormatter
alongside the plain-text part in a multipart/alternative message.

- EmailConfig gains html: bool field
- mailer.build_message/send_report accept rows for HTML generation
- CLI passes rows to send_report
- --init-config generates email.html: false
- README.md and docs/CONFIG.md updated

Bump version to 1.10.0.

Closes #48

redmine_reporter/cli.py

@@ -76,6 +76,7 @@ def _run_init_config(config_path: str, force: bool) -> int:
             "no_time": False,
         },
         "email": {
+            "html": False,
             "smtp": {
                 "host": "",
                 "port": 587,
@@ -209,7 +210,13 @@ def _save_and_maybe_send(
             return 1
 
         try:
-            send_report(email_config, output_arg, author, f"{from_date}--{to_date}")
+            send_report(
+                email_config,
+                output_arg,
+                author,
+                f"{from_date}--{to_date}",
+                rows=rows,
+            )
             print(f"📧 Report sent to {', '.join(email_config.to)}")
         except RedmineAPIError as e:
             print(f"❌ {e.message}", file=sys.stderr)