feat(workflows): React Flow canvas editor + list + approvals UI

Adds the admin Workflows surface (top-level nav, gated by the workflows
flag + workflows.view): a list page (enable toggle, delete, new), a
pending-approvals inbox (confirm/deny), and a React Flow (@xyflow/react)
canvas editor — palette to add nodes, drag handle→handle to connect
(branch/gate/loop expose yes-no / confirm-deny / loop-exit handles), a
side-panel JSON config editor, and save (writes a new version). Routes +
sidebar entry + workflows.service. Build + tsc clean.

NOTE: the workflow page strings render via inline English fallbacks; DE
translations for the workflows.* block are still pending native review.

frontend/package.json

@@ -32,6 +32,7 @@
     "@types/dompurify": "^3.0.5",
     "@types/lodash": "^4.17.20",
     "@types/react-google-recaptcha": "^2.1.9",
+    "@xyflow/react": "^12.11.1",
     "axios": "1.15.2",
     "clsx": "^2.0.0",
     "date-fns": "4.1.0",