Add multi-user accounts with per-user data scoping and 2FA

Projects were previously global to anyone who could reach the server.
Adds email/password accounts with httpOnly cookie sessions, scopes
every project (both SQLite and localStorage) to the signed-in user,
and auto-claims pre-existing unowned projects for whoever registers
first. Also adds optional TOTP two-factor auth with backup codes,
managed from a new Account Settings page, since there's no
password-reset flow to fall back on otherwise.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

server/sessionStore.js

@@ -0,0 +1,48 @@
+import session from 'express-session';
+import { getSession, setSession, destroySession, pruneExpiredSessions } from './db.js';
+
+const DAY_MS = 24 * 60 * 60 * 1000;
+
+/**
+ * express-session store backed by the same SQLite database as everything else,
+ * so logins survive a server restart without adding another dependency.
+ */
+export class SqliteSessionStore extends session.Store {
+  constructor() {
+    super();
+    // Sweep expired sessions periodically instead of on every request.
+    this._interval = setInterval(() => pruneExpiredSessions(), DAY_MS);
+    this._interval.unref?.();
+  }
+
+  get(sid, cb) {
+    try {
+      cb(null, getSession(sid));
+    } catch (err) {
+      cb(err);
+    }
+  }
+
+  set(sid, sessionData, cb) {
+    try {
+      const maxAge = sessionData.cookie?.maxAge ?? DAY_MS * 30;
+      setSession(sid, sessionData, Date.now() + maxAge);
+      cb?.(null);
+    } catch (err) {
+      cb?.(err);
+    }
+  }
+
+  destroy(sid, cb) {
+    try {
+      destroySession(sid);
+      cb?.(null);
+    } catch (err) {
+      cb?.(err);
+    }
+  }
+
+  touch(sid, sessionData, cb) {
+    this.set(sid, sessionData, cb);
+  }
+}