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>

scripts/dev.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Start backend and frontend for local development.
+# Backend runs with --dev (auth disabled), frontend points at localhost:8080.
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+
+cleanup() {
+    echo ""
+    echo "Shutting down..."
+    kill $BACKEND_PID $FRONTEND_PID 2>/dev/null || true
+    wait $BACKEND_PID $FRONTEND_PID 2>/dev/null || true
+}
+trap cleanup EXIT INT TERM
+
+# Start backend
+echo "Starting backend (dev mode) on :8080..."
+cd "$ROOT/opal-task"
+go run . server start --dev --addr :8080 &
+BACKEND_PID=$!
+
+# Give the backend a moment to start before launching the frontend
+sleep 1
+
+# Start frontend
+echo "Starting frontend..."
+cd "$ROOT/opal-web"
+bun run dev &
+FRONTEND_PID=$!
+
+wait