fix: resolve trainers page issues and add seed data

- Add SQL seed data for trainers and collections
- Fix React hydration warning with suppressHydrationWarning
- Add empty state UI with friendly messaging
- Add error state UI with retry functionality
- Link Add Trainer buttons to /trainers/new page

supabase/seed.sql

@@ -0,0 +1,52 @@
+-- ============================================================
+-- TabataFit Seed Data
+-- ============================================================
+-- Run this SQL in Supabase SQL Editor to populate your database with sample data
+
+-- Sample Trainers
+INSERT INTO public.trainers (name, specialty, color, workout_count) VALUES
+  ('Sarah Chen', 'HIIT & Cardio', '#FF6B35', 12),
+  ('Marcus Johnson', 'Strength Training', '#5AC8FA', 8),
+  ('Elena Rodriguez', 'Yoga & Mobility', '#30D158', 15),
+  ('David Kim', 'Full Body Conditioning', '#FF9500', 10);
+
+-- Sample Collections
+INSERT INTO public.collections (title, description, icon, gradient) VALUES
+  ('Quick Burns', 'High-intensity workouts under 10 minutes', 'flame', ARRAY['#FF6B35', '#FF9500']),
+  ('Core Strength', 'Build a solid foundation', 'shield', ARRAY['#5AC8FA', '#30D158']),
+  ('Cardio Blast', 'Get your heart pumping', 'heart', ARRAY['#FF6B35', '#FF3B30']),
+  ('Total Body', 'Full body workouts for maximum results', 'body', ARRAY['#30D158', '#5856D6']);
+
+-- Note: Workouts will be added after trainers are created (they reference trainer_ids)
+-- To add workouts, first note the UUIDs generated for trainers above, then:
+
+-- Sample Workout (uncomment and update trainer_id after running above)
+-- INSERT INTO public.workouts (
+--   title, trainer_id, category, level, duration, calories, rounds,
+--   prep_time, work_time, rest_time, equipment, music_vibe, exercises, is_featured
+-- ) VALUES (
+--   'Morning Ignite',
+--   'PASTE_TRAINER_UUID_HERE',
+--   'full-body',
+--   'Beginner',
+--   4,
+--   45,
+--   8,
+--   10,
+--   20,
+--   10,
+--   ARRAY['yoga-mat'],
+--   'electronic',
+--   '[{"name": "Jumping Jacks", "duration": 20}, {"name": "Push-ups", "duration": 20}, {"name": "Squats", "duration": 20}, {"name": "Plank", "duration": 20}]'::jsonb,
+--   true
+-- );
+
+-- Update trainer workout counts
+UPDATE public.trainers SET workout_count = 12 WHERE name = 'Sarah Chen';
+UPDATE public.trainers SET workout_count = 8 WHERE name = 'Marcus Johnson';
+UPDATE public.trainers SET workout_count = 15 WHERE name = 'Elena Rodriguez';
+UPDATE public.trainers SET workout_count = 10 WHERE name = 'David Kim';
+
+-- Verify data
+SELECT * FROM public.trainers ORDER BY name;
+SELECT * FROM public.collections ORDER BY title;