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/tests/mail-store.test.ts

@@ -113,6 +113,33 @@ describe("MailStore.entries", () => {
 		expect(entries[0].kind).toBe("standalone");
 	});
 
+	it("collapses fan-out rows for the same broadcast message_id", () => {
+		// `/sent` returns one row per recipient for broadcast and admin
+		// sends so the admin tooling can render the materialised
+		// audience. The list pane must collapse the duplicates by
+		// message_id, otherwise the {#each} key in `thread-list.svelte`
+		// repeats and Svelte 5 surfaces `each_key_duplicate`.
+		const store = new MailStore();
+		const base = {
+			messageId: "br-shared",
+			senderKind: "player" as const,
+			senderRaceName: "Self",
+			broadcastScope: "game_broadcast" as const,
+			createdAt: new Date("2026-05-15T14:00:00Z"),
+		};
+		store.sent = [
+			makeMessage({ ...base, recipientRaceName: "Greys" }),
+			makeMessage({ ...base, recipientRaceName: "Bajori" }),
+			makeMessage({ ...base, recipientRaceName: "Romulans" }),
+		];
+
+		const entries = store.entries;
+		expect(entries.length).toBe(1);
+		expect(entries[0].kind).toBe("standalone");
+		const standalone = entries[0] as MailStandalone;
+		expect(standalone.message.messageId).toBe("br-shared");
+	});
+
 	it("counts unread incoming messages in unreadCount", () => {
 		const store = new MailStore();
 		store.inbox = [