Indoor / Outdoor training mode toggle

Let IntervalCoach know whether a session is done indoors (smart trainer) or outdoors, and tailor the workout to that environment.

Indoor mode would lean into what a trainer does well:

• Shorter, more tightly structured intervals (ERG mode lets you hit exact power)
• More on-screen coaching cues during the workout (e.g. messages shown on Zwift mid-interval)

Outdoor mode would stay flexible — wider power ranges or HR targets and longer, less rigid efforts that survive wind, terrain and traffic.

Ideally this is a per-day preference (e.g. Tue/Thu = trainer, Sat/Sun = outside) so the planner can place precision interval work on indoor days and flexible endurance work on outdoor days.