Add --dev flag to `opal server start` that disables auth (injects
userID=1 for all requests) and exposes a /auth/dev-session endpoint,
so the frontend can develop against a real backend without OAuth
config. Remove VITE_MOCK_MODE and all mock data/branches from the
frontend stores. Add scripts/dev.sh to start both services locally.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
