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/css/theme.css

@@ -0,0 +1,33 @@
+/* Field Ops Portal theme. Every colour and font the site uses comes from here. */
+:root {
+  --color-brand: #0f766e;
+  --color-brand-strong: #115e59;
+  --color-brand-tint: #ccfbf1;
+  --color-bg: #f8fafc;
+  --color-surface: #ffffff;
+  --color-surface-alt: #f1f5f9;
+  --color-border: #e2e8f0;
+  --color-text: #0f172a;
+  --color-text-muted: #64748b;
+  --color-success: #15803d;
+  --color-warning: #b45309;
+  --color-danger: #b91c1c;
+  --font-sans: "Trebuchet MS", "Segoe UI", sans-serif;
+  --font-heading: Georgia, "Times New Roman", serif;
+  --radius: 6px;
+}
+
+[data-theme="dark"] {
+  --color-brand: #0d9488;
+  --color-brand-strong: #134e4a;
+  --color-brand-tint: #134e4a;
+  --color-bg: #0b1120;
+  --color-surface: #111827;
+  --color-surface-alt: #1f2937;
+  --color-border: #334155;
+  --color-text: #e2e8f0;
+  --color-text-muted: #94a3b8;
+  --color-success: #22c55e;
+  --color-warning: #f59e0b;
+  --color-danger: #ef4444;
+}