refactor: consolidate blockchain explorer into single app and update backup ignore patterns

- Remove standalone explorer-web app (README, HTML, package files)
- Add /web endpoint to blockchain-explorer for web interface access
- Update .gitignore to exclude application backup archives (*.tar.gz, *.zip)
- Add backup documentation files to .gitignore (BACKUP_INDEX.md, README.md)
- Consolidate explorer functionality into main blockchain-explorer application

apps/marketplace/src/lib/auth.ts

@@ -0,0 +1,33 @@
+export interface MarketplaceSession {
+  token: string;
+  expiresAt: number;
+}
+
+const STORAGE_KEY = "marketplace-session";
+
+export function saveSession(session: MarketplaceSession): void {
+  localStorage.setItem(STORAGE_KEY, JSON.stringify(session));
+}
+
+export function loadSession(): MarketplaceSession | null {
+  const raw = localStorage.getItem(STORAGE_KEY);
+  if (!raw) {
+    return null;
+  }
+  try {
+    const data = JSON.parse(raw) as MarketplaceSession;
+    if (typeof data.token === "string" && typeof data.expiresAt === "number") {
+      if (data.expiresAt > Date.now()) {
+        return data;
+      }
+      clearSession();
+    }
+  } catch (error) {
+    console.warn("Failed to parse stored marketplace session", error);
+  }
+  return null;
+}
+
+export function clearSession(): void {
+  localStorage.removeItem(STORAGE_KEY);
+}