Timer for Timed Exercises

[tbouri]

Proposed Changes

• Added unit_type field to RepetitionUnit model to distinguish TIME, REPETITIONS, and DISTANCE units
• Created "Add Timed Exercise" form (Training > Add Timed Exercise) allowing users to add time-based exercises to routines
• Implemented countdown timer that appears as a stopwatch button next to exercises with time units (Seconds/Minutes) on routine view pages
• Timer modal includes circular progress display, start/pause/reset controls, and audio completion alert

How it works

1. Navigate to Training > Add Timed Exercise
2. Select a routine, exercise (e.g., Plank), duration, and unit (Seconds/Minutes)
3. View the routine - a stopwatch icon appears next to the timed exercise
4. Click the icon to open the countdown timer
5. Use start/pause/reset controls; audio plays on completion

Related Issue(s)

See also #2008

User commands due to this PR

• Users need to run database migrations
  • This applies the migration that adds the unit_type field to RepetitionUnit and sets existing units to their appropriate types (Seconds/Minutes → TIME, Miles/Km → DISTANCE, Reps/Till Failure → REPETITIONS).