fix(ui): F8-04 profile polish — IANA timezone picker, save-stay, shared identity cache

PR-feedback round on #60:

- Time-zone field is now a continent-grouped <select> populated from
  `Intl.supportedValuesOf("timeZone")`, with the browser-detected
  zone pre-selected when no value is stored. A stored zone the
  runtime no longer advertises is preserved as an "Other" entry.
- Saving the profile no longer kicks the user back to the lobby:
  the form stays put and shows a transient `saved` notice, cleared
  on the next edit. Only `cancel` returns to the lobby.
- New `lib/account-store.svelte.ts` caches `user.account.get` for
  the session; lobby + profile share it through `account.ensure()`,
  so navigating Overview ⇄ Profile no longer flashes the
  "loading account…" placeholder or fires a second gateway call.
  Profile save writes through to the store so the shell identity
  strip picks up the new display name without refetching. Cleared
  on logout to prevent identity bleed between accounts.
- e2e: existing 4 cases adjusted for save-stay; added two new ones
  for the timezone dropdown and identity-strip stability across
  navigation.
- Docs: `ui/docs/lobby.md` updated to describe the shared cache,
  the new timezone picker shape, and the save-stay behaviour.

ui/frontend/src/lib/screens/lobby-shell.svelte

@@ -4,23 +4,27 @@ landing and the editable profile. Renders a left page-list sidebar
 (mirroring the project site's VitePress layout) plus a top identity
 strip ("Player-xxxx" → opens profile, logout). Children fill the
 right-hand column. Pages mark themselves active via `activePage`.
+
+The identity strip reads directly from the session-wide `account`
+store so navigating Overview ⇄ Profile never re-renders an empty
+placeholder: both screens populate the same cache through
+`account.ensure(client)` and the shell renders the latest value.
 -->
 <script lang="ts">
 	import type { Snippet } from "svelte";
 	import { appScreen } from "$lib/app-nav.svelte";
 	import { i18n } from "$lib/i18n/index.svelte";
 	import { session } from "$lib/session-store.svelte";
+	import { account } from "$lib/account-store.svelte";
 
 	type Page = "overview" | "profile";
 
 	interface Props {
 		activePage: Page;
-		displayName: string;
-		userName: string;
 		children: Snippet;
 	}
 
-	let { activePage, displayName, userName, children }: Props = $props();
+	let { activePage, children }: Props = $props();
 
 	const PAGES: ReadonlyArray<{ id: Page; labelKey: "lobby.nav.overview" | "lobby.nav.profile"; screen: "lobby" | "profile" }> = [
 		{ id: "overview", labelKey: "lobby.nav.overview", screen: "lobby" },
@@ -28,9 +32,12 @@ right-hand column. Pages mark themselves active via `activePage`.
 	];
 
 	let identityLabel = $derived.by(() => {
-		const trimmed = displayName.trim();
-		if (trimmed.length > 0) return trimmed;
-		if (userName.length > 0) return userName;
+		const current = account.current;
+		if (current !== null) {
+			const trimmed = current.displayName.trim();
+			if (trimmed.length > 0) return trimmed;
+			if (current.userName.length > 0) return current.userName;
+		}
 		return i18n.t("lobby.account_loading");
 	});