Add Workout Presets & Custom Mode

Read plan.md. Build milestone: Workout Presets.

Add preset selection above the timer (visible only when timer is idle):

- 4 preset cards in horizontal scroll row:
  - Beginner: 20s run / 40s walk, 8 rounds, 60s warmup/cooldown 🌱
  - Fat Burn: 30s run / 30s walk, 12 rounds, 60s warmup/cooldown 🔥
  - HIIT Beast: 40s run / 20s walk, 15 rounds, 30s warmup/cooldown ⚡
  - Custom: user-defined ⚙️

- Selected preset has highlighted border (#6C63FF)
- Selecting Custom shows +/- stepper controls for: run time, walk time, rounds, warmup, cooldown
- Show workout summary card below presets: total duration, colored tags for each phase
- Timer uses selected preset values when started