Split monolithic index.html and style.css for maintainability

- Extract 15 modals and 3 partials from index.html into Jinja2 templates
  (templates/modals/*.html, templates/partials/*.html)
- Split style.css (3,712 lines) into 11 feature-scoped CSS files under
  static/css/ (base, layout, components, cards, modal, calibration,
  dashboard, streams, patterns, profiles, tutorials)
- Switch root route from FileResponse to Jinja2Templates
- Add jinja2 dependency
- Consolidate duplicate @keyframes spin definition
- Browser receives identical assembled HTML — zero JS changes

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

server/pyproject.toml

@@ -36,6 +36,7 @@ dependencies = [
     "python-json-logger>=3.1.0",
     "python-dateutil>=2.9.0",
     "python-multipart>=0.0.12",
+    "jinja2>=3.1.0",
     "wmi>=1.5.1; sys_platform == 'win32'",
     "zeroconf>=0.131.0",
     "pyserial>=3.5",