Switch to mailparser, UID watermarking, and 45s polling

- Replace hand-rolled MIME parsing with mailparser's simpleParser
- Use HTML body in prompts for better encoding/structure fidelity
- Replace IMAP IDLE (unreliable on Loopia) with 45s polling loop
- Track highest UID on startup; only fetch uid > last_uid each poll
  to avoid re-transferring old messages
- Add node: prefix to built-in imports

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -3,6 +3,7 @@
 	"version": "0.1.0",
 	"type": "module",
 	"dependencies": {
-		"imapflow": "^1.0.0"
+		"imapflow": "^1.0.0",
+		"mailparser": "^3.9.4"
 	}
-}
+}