Port MAIL in the party menu and the PC's MAIL BOX

Giving MAIL opens the mail entry: two lines of up to sixteen characters,
kept with the player's name and ID, the holder's species and the MAIL.
The party submenu's MAIL reads the letter or takes it, either to the PC's
MAILBOX (ten at most) or into the pack without its message. The player's
PC lists the MAILBOX by author to read, put in the pack or attach, and
Bill's PC keeps a Pokémon holding MAIL out of the boxes. Saves keep each
holder's MAIL and the MAILBOX.

The letter shows its message and author natively; the stationery's
graphics are not drawn yet.

Closes #27

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/game/party/pokemon.ts

@@ -1,4 +1,5 @@
 import type { PokemonData } from "../rom/pokemon";
+import type { Mail } from "./mail";
 
 /** A party Pokémon with the fields of the original party_struct. */
 export interface PartyMon {
@@ -24,6 +25,8 @@ export interface PartyMon {
   hp: number;
   /** Max HP, Attack, Defense, Speed, Sp. Atk, Sp. Def. */
   stats: [number, number, number, number, number, number];
+  /** sPartyMail's entry while the Pokémon holds MAIL. */
+  mail?: Mail;
 }
 
 export const PARTY_LENGTH = 6;