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/README.md

@@ -0,0 +1,10 @@
+# Fixture (c): web project with an existing frontend
+
+A plain HTML site in `web/` with its own teal theme: light and dark tokens in
+`web/css/theme.css`, layout in `web/css/app.css`, logic in `web/js/app.js`. No bot.
+
+The no-AI skill should report this frontend (location, stack, colours), ask keep or
+restyle, never ask desktop or web, and build a dashboard page on sample data.
+- Keep: no colour question; the page uses the theme's tokens and fonts; no existing
+  file changes.
+- Restyle: asks for colours; only token lines in the existing files change.