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>

deploy/study-app.service

@@ -10,6 +10,9 @@ Group=study-app
 WorkingDirectory=/opt/study-app
 Environment=NODE_ENV=production
 Environment=PORT=3001
+# Replaced with a generated value by install.sh — keep this secret and stable
+# across deploys, or every existing login session gets invalidated.
+Environment=SESSION_SECRET=__SESSION_SECRET__
 ExecStart=/usr/bin/node server/index.js
 Restart=on-failure
 RestartSec=5