Existing frontend: detect, report, then keep or restyle (ticket 03)

- templates/detect_frontend.py: finds plain HTML, React/Vue/Angular/Svelte apps
  and PyQt web views; prints location, stack, main colours, font and every
  token block (CSS custom properties, <style> tags, Tailwind config).
- templates/frontend.md: shared report / keep / restyle procedure, written
  so rpa-dashboard-ai can point to it.
- SKILL.md steps 1, 2 and 6 point to it.
- tests: detection self-check; fixture c-web-existing-frontend; eval entries
  c-keep and c-restyle (new pages only, per-question grading, checks on what
  changed in the existing files), both passing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/fixtures/c-web-existing-frontend/web/js/app.js

@@ -0,0 +1,15 @@
+// Field Ops Portal: job list and dark toggle.
+const JOBS = [
+  { job: "J-101", site: "North depot", status: "Done", tag: "ok" },
+  { job: "J-102", site: "Harbour road", status: "Late", tag: "late" },
+  { job: "J-103", site: "West yard", status: "Failed", tag: "failed" },
+];
+
+document.getElementById("jobs").innerHTML = JOBS.map(
+  (j) => `<tr><td>${j.job}</td><td>${j.site}</td><td><span class="tag ${j.tag}">${j.status}</span></td></tr>`
+).join("");
+
+document.getElementById("theme-btn").onclick = () => {
+  const root = document.documentElement;
+  root.dataset.theme = root.dataset.theme === "dark" ? "light" : "dark";
+};