UI: unified design system, light/dark themes, redesigned pages and player UI

Tokens and components in styles.css, shared Ui (dialogs, toasts), icons,
bottom tab bar on mobile, keyboard/gamepad navigation, library shelf with
filters and conversion badges, hero game page, restyled player screens.
Games list now returns per-game version counts. Adds docs/UI_DESIGN.md.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

frontend/src/recent.ts

@@ -0,0 +1,37 @@
+// "Continue playing": remembered per browser, the server stores only saves.
+export interface RecentPlay {
+  gameId: number;
+  versionId: number;
+  title: string;
+  coverHash: string | null;
+  at: number;
+}
+
+const KEY = "pg-recent";
+const MAX = 12;
+
+export function getRecent(): RecentPlay[] {
+  try {
+    const list = JSON.parse(localStorage.getItem(KEY) ?? "[]");
+    return Array.isArray(list) ? list : [];
+  } catch {
+    return [];
+  }
+}
+
+export function pushRecent(item: Omit<RecentPlay, "at">) {
+  try {
+    const list = getRecent().filter((r) => r.gameId !== item.gameId);
+    localStorage.setItem(KEY, JSON.stringify([{ ...item, at: Date.now() }, ...list].slice(0, MAX)));
+  } catch {
+    /* storage unavailable */
+  }
+}
+
+export function dropRecent(gameId: number) {
+  try {
+    localStorage.setItem(KEY, JSON.stringify(getRecent().filter((r) => r.gameId !== gameId)));
+  } catch {
+    /* storage unavailable */
+  }
+}