feat: replace mock mode with real backend dev mode

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>

opal-web/.env.example

@@ -2,5 +2,5 @@
 VITE_API_URL=https://opal.example.com/api
 VITE_AUTH_URL=https://auth.example.com
 
-# OAuth
+# OAuth — set to "false" for local dev (used with `opal server start --dev`)
 VITE_OAUTH_ENABLED=true