mail UI: dedupe broadcast fan-out and drop in-game admin compose

Two issues surfaced once the long-lived dev environment finally
reached the diplomail view:

1. `/sent` returns one row per recipient for broadcast and admin
   fan-outs (so the admin tooling can render the materialised
   audience). The list pane fed all rows into the stand-alone
   bucket, so the `{#each entries as e (entryKey(e))}` key in
   `thread-list.svelte` collapsed to the same `standalone:${id}`
   for every recipient and Svelte 5 aborted the render with
   `each_key_duplicate`. Dedupe stand-alones by `message_id` in
   `buildEntries`.

2. The compose dialog exposed an `admin` kind toggle gated on
   "owner of game". That was a Phase 28 plan decision, but admin
   compose is an operator tool (server admin), not an in-game
   action — every game owner should not be able to broadcast
   admin notifications. Drop the admin option, the audience
   sub-toggles, and the admin path through `submit`. The
   `MailStore.composeAdmin` wrapper and the backend RPC stay so
   the future admin UI can call them.

Vitest covers the fan-out dedup with three rows sharing one
`message_id` collapsing to a single stand-alone entry.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

ui/frontend/src/lib/mail-store.svelte.ts

@@ -317,9 +317,24 @@ function buildEntries(inbox: MailMessage[], sent: MailMessage[]): MailListEntry[
 		thread.latestAt = last.createdAt;
 	}
 
+	// Broadcast and admin fan-outs return one row per recipient from
+	// the `/sent` endpoint (so the admin UI sees the materialised
+	// audience). The in-game list pane collapses them by `message_id`
+	// — without this dedupe the {#each} key in `thread-list.svelte`
+	// repeats and Svelte 5 aborts the render with `each_key_duplicate`.
+	const seen = new Set<string>();
+	const dedupedStandalones: MailStandalone[] = [];
+	for (const s of standalones) {
+		if (seen.has(s.message.messageId)) {
+			continue;
+		}
+		seen.add(s.message.messageId);
+		dedupedStandalones.push(s);
+	}
+
 	const entries: MailListEntry[] = [
 		...Array.from(threadsByRace.values()),
-		...standalones,
+		...dedupedStandalones,
 	];
 	entries.sort((a, b) => b.latestAt.getTime() - a.latestAt.getTime());
 	return entries;