Phase 28 (Step 11): Vitest coverage for MailStore threading

`tests/mail-store.test.ts` exercises the `entries` derived rune
with handcrafted inbox + sent fixtures:

- personal messages exchanged with one race collapse into a
  per-race thread with messages sorted oldest → newest;
- system mail (`sender_kind=system`) and admin notifications
  (`sender_kind=admin`) surface as stand-alone items even when a
  race-name snapshot is present;
- the caller's own paid-tier broadcasts (`broadcast_scope=
  game_broadcast`) render as stand-alone outgoing items;
- `unreadCount` counts inbox rows with `readAt === null`.

The store fields are mutated directly to avoid wiring a fake
`GalaxyClient`; the underlying `$derived` rune fires whenever
those fields change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/frontend/tests/mail-store.test.ts

@@ -0,0 +1,125 @@
+// Phase 28 — MailStore threading projection tests. Exercises the
+// `entries` derived rune end-to-end with handcrafted inbox + sent
+// fixtures; the network surface (GalaxyClient) is left null since
+// these tests do not call init / setGame.
+
+import { describe, expect, it } from "vitest";
+
+import {
+	MailStore,
+	type MailListEntry,
+	type MailStandalone,
+	type MailThread,
+} from "../src/lib/mail-store.svelte";
+import type { MailMessage } from "../src/api/diplomail";
+
+function makeMessage(overrides: Partial<MailMessage>): MailMessage {
+	return {
+		messageId: overrides.messageId ?? crypto.randomUUID(),
+		gameId: overrides.gameId ?? "00000000-0000-0000-0000-000000000001",
+		gameName: overrides.gameName ?? "Test game",
+		kind: overrides.kind ?? "personal",
+		senderKind: overrides.senderKind ?? "player",
+		senderUserId: overrides.senderUserId ?? null,
+		senderUsername: overrides.senderUsername ?? null,
+		senderRaceName: overrides.senderRaceName ?? null,
+		subject: overrides.subject ?? "",
+		body: overrides.body ?? "",
+		bodyLang: overrides.bodyLang ?? "en",
+		broadcastScope: overrides.broadcastScope ?? "single",
+		createdAt: overrides.createdAt ?? new Date(0),
+		recipientUserId:
+			overrides.recipientUserId ?? "00000000-0000-0000-0000-000000000099",
+		recipientUserName: overrides.recipientUserName ?? "self",
+		recipientRaceName: overrides.recipientRaceName ?? null,
+		readAt: overrides.readAt ?? null,
+		deletedAt: overrides.deletedAt ?? null,
+		translatedSubject: overrides.translatedSubject ?? null,
+		translatedBody: overrides.translatedBody ?? null,
+		translationLang: overrides.translationLang ?? null,
+		translator: overrides.translator ?? null,
+	};
+}
+
+describe("MailStore.entries", () => {
+	it("groups inbox + sent messages into per-race threads", () => {
+		const store = new MailStore();
+		const greysIncoming = makeMessage({
+			messageId: "m1",
+			senderKind: "player",
+			senderRaceName: "Greys",
+			body: "hi",
+			createdAt: new Date("2026-05-15T12:00:00Z"),
+		});
+		const greysOutgoing = makeMessage({
+			messageId: "m2",
+			senderKind: "player",
+			senderRaceName: "Self",
+			recipientRaceName: "Greys",
+			body: "reply",
+			createdAt: new Date("2026-05-15T12:05:00Z"),
+		});
+		store.inbox = [greysIncoming];
+		store.sent = [greysOutgoing];
+
+		const entries = store.entries;
+		expect(entries.length).toBe(1);
+		const entry = entries[0];
+		expect(entry.kind).toBe("thread");
+		const thread = entry as MailThread;
+		expect(thread.raceName).toBe("Greys");
+		expect(thread.messages.map((m) => m.messageId)).toEqual(["m1", "m2"]);
+	});
+
+	it("surfaces system and admin messages as stand-alone items", () => {
+		const store = new MailStore();
+		const system = makeMessage({
+			messageId: "sys-1",
+			kind: "admin",
+			senderKind: "system",
+			subject: "game.paused: details",
+			createdAt: new Date("2026-05-15T13:00:00Z"),
+		});
+		const admin = makeMessage({
+			messageId: "adm-1",
+			kind: "admin",
+			senderKind: "admin",
+			createdAt: new Date("2026-05-15T13:05:00Z"),
+		});
+		store.inbox = [system, admin];
+
+		const entries: MailListEntry[] = store.entries;
+		expect(entries.every((e) => e.kind === "standalone")).toBe(true);
+		const standalones = entries as MailStandalone[];
+		expect(standalones.map((e) => e.message.messageId).sort()).toEqual(
+			["adm-1", "sys-1"],
+		);
+	});
+
+	it("treats the local player's broadcasts as stand-alone outgoing", () => {
+		const store = new MailStore();
+		const broadcast = makeMessage({
+			messageId: "br-1",
+			senderKind: "player",
+			senderRaceName: "Self",
+			recipientRaceName: "Greys",
+			broadcastScope: "game_broadcast",
+			createdAt: new Date("2026-05-15T14:00:00Z"),
+		});
+		store.sent = [broadcast];
+
+		const entries = store.entries;
+		expect(entries.length).toBe(1);
+		expect(entries[0].kind).toBe("standalone");
+	});
+
+	it("counts unread incoming messages in unreadCount", () => {
+		const store = new MailStore();
+		store.inbox = [
+			makeMessage({ readAt: null, createdAt: new Date(1) }),
+			makeMessage({ readAt: new Date(2), createdAt: new Date(2) }),
+			makeMessage({ readAt: null, createdAt: new Date(3) }),
+		];
+		expect(store.unreadCount).toBe(2);
+	});
+});