Plan & Scaffold the Django Project

Create a plan for building a Cardio Interval Trainer PWA app served by Django.

Save the plan as plan.md with:
- App overview: a mobile-first interval timer for cardio workouts (run/walk cycles)
- Feature list broken into milestones
- Tech decisions: Django backend, single-page template, vanilla JS, localStorage for data, PWA with service worker
- UI specs: dark theme, mobile-first (420px max), colors (bg #0a0a1a, primary #6C63FF, run #FF4D4D, walk #4DFFB5, warmup #FFB84D, cooldown #4DB8FF), fonts (DM Sans + JetBrains Mono from Google Fonts)

Then scaffold the Django project:
- Project: "cardioflow", app: "trainer"
- Single view serving one template at "/"
- Static files structure ready
- Empty base template with proper HTML head (viewport, theme-color, PWA meta tags, fonts loaded)
- Runnable with python manage.py runserver