feat(C3): add React Flow workflow editor

frontend/src/pages/WorkflowEditor.tsx: full React Flow editor with
custom nodes (Input/Convert/Render/FFmpeg/Output), config sidepanel,
node palette with drag-drop, new workflow dialog.
frontend/src/api/workflows.ts: workflow CRUD API client.
@xyflow/react added to package.json dependencies.
Route /workflows + sidebar link for admin+pm.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/layout/Layout.tsx

@@ -1,5 +1,5 @@
 import { Outlet, NavLink, useNavigate, Link } from 'react-router-dom'
-import { LayoutDashboard, Package, Settings, LogOut, FlaskConical, Activity, Library, Plus, SlidersHorizontal, Building2 } from 'lucide-react'
+import { LayoutDashboard, Package, Settings, LogOut, FlaskConical, Activity, Library, Plus, SlidersHorizontal, Building2, GitBranch } from 'lucide-react'
 import { useAuthStore } from '../../store/auth'
 import { clsx } from 'clsx'
 import { useQuery } from '@tanstack/react-query'
@@ -120,6 +120,22 @@ export default function Layout() {
               Admin
             </NavLink>
           )}
+          {(user?.role === 'admin' || user?.role === 'project_manager') && (
+            <NavLink
+              to="/workflows"
+              className={({ isActive }) =>
+                clsx(
+                  'flex items-center gap-3 px-3 py-2 rounded-md text-sm font-medium transition-colors',
+                  isActive
+                    ? 'bg-accent-light text-accent'
+                    : 'text-content-secondary hover:bg-surface-hover',
+                )
+              }
+            >
+              <GitBranch size={18} />
+              Workflows
+            </NavLink>
+          )}
           {user?.role === 'admin' && (
             <NavLink
               to="/tenants"