Modernize legacy dependencies

[coder13]

Summary

• replace the abandoned express-socket.io-session bridge with direct Express session middleware shared by Socket.IO
• upgrade MongoDB to 7, Mongoose to 8, and the session store to connect-mongo 6
• move the client to React 18.3 and MUI 6, remove connected-react-router, and replace react-ga with GA4-compatible react-ga4
• upgrade Jest, Testing Library, ESLint, and Husky while removing the leftover CRA and Enzyme configuration
• add Dependabot configuration and CodeQL analysis

Why

Several core dependencies were abandoned, end-of-life, or old enough to prevent security and framework maintenance. The existing Socket.IO session bridge also pulled in a vulnerable cookie dependency, while the Universal Analytics package could not correctly use the configured GA4 property.

Impact

The client remains on React 18.3 as the compatibility bridge before React 19. Existing room, authentication, and Socket.IO behavior is preserved. MongoDB 7 is used as the safe intermediate data upgrade before an eventual MongoDB 8 move.

The migration also fixes Vite loading for the protocol/event modules and updates the lobby layout for MUI 6.

Validation

• yarn install --frozen-lockfile
• yarn lint
• yarn test — 31 tests passed
• yarn build
• Cypress — 6 browser tests passed against MongoDB 7 and Redis
• docker compose config -q
• yarn audit — 0 vulnerabilities across 538 packages
• git diff --check

Follow-ups

• migrate remaining local function-component defaultProps and replace React-18-only leaf dependencies before React 19
• deploy through MongoDB 7 and update feature compatibility before moving existing volumes to MongoDB 8
• enable GitHub-native secret scanning in repository settings

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