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/ui/shell.ts

@@ -32,6 +32,7 @@ export function renderShell(root: HTMLElement) {
                 <div class="battle-box"><p id="battle-text" aria-live="polite"></p><div id="battle-menu" class="battle-menu"></div></div>
               </div>
               <div id="stats-screen" class="stats-screen" hidden aria-label="Pokémon stats"><div class="stats-top"><canvas id="stats-pic" width="56" height="56" role="img"></canvas><div id="stats-header" class="stats-header"></div></div><div id="stats-page" class="stats-page" aria-live="polite"></div><div class="stats-nav"><button id="stats-prev" aria-label="Previous page">◀</button><span id="stats-page-number"></span><button id="stats-next" aria-label="Next page">▶</button><button id="stats-close">Close</button></div></div>
+              <div id="mail-view" class="mail-view" hidden role="dialog" aria-labelledby="mail-type"><p id="mail-type" class="mail-type"></p><p id="mail-message" class="mail-message"></p><p id="mail-author" class="mail-author"></p><button id="mail-close">Close</button></div>
               <div id="pokegear" class="pokegear" hidden aria-label="Pokégear"><div id="pokegear-cards" class="pokegear-cards" aria-label="Cards"></div><div id="pokegear-card" class="pokegear-card" aria-live="polite"></div></div>
               <div id="trainer-card" class="trainer-card" hidden aria-label="Trainer card"><div class="trainer-card-top"><dl id="trainer-card-info"></dl><canvas id="trainer-card-pic" width="40" height="56" role="img"></canvas></div><div id="trainer-card-page" class="trainer-card-page"></div></div>
               <div id="options" class="options-screen" hidden aria-label="Options"><ol id="options-list"></ol></div>