Add training tracking: exercises with instructions, one-tap session log, consistency overview

Exercises (name + how-to note) are a new synced collection with the same
LWW/tombstone contract as events, served by POST /api/exercises/sync.
Training sessions are ordinary events (type "training") referencing an
exercise by id, so they ride the existing event sync unchanged.

The Training panel lists each exercise with last-trained / this-week /
streak stats, expandable instructions, and a one-tap Log button with the
usual undo/add-note snackbar. An exercise-by-day heatmap shows the last
14 days of consistency, and history and the daily overview count
training sessions like any other event.

server/auth.go

@@ -173,6 +173,9 @@ func (a *Auth) adopt(userID string) error {
 	if _, err := a.db.Exec(`UPDATE config SET user_id = ? WHERE user_id = ''`, userID); err != nil {
 		return err
 	}
+	if _, err := a.db.Exec(`UPDATE exercises SET user_id = ? WHERE user_id = ''`, userID); err != nil {
+		return err
+	}
 	return a.adoptPhotos(userID)
 }
 
@@ -402,6 +405,7 @@ func (a *Auth) deleteAccount(userID string) error {
 	defer tx.Rollback()
 	for _, q := range []string{
 		`DELETE FROM events WHERE user_id = ?`,
+		`DELETE FROM exercises WHERE user_id = ?`,
 		`DELETE FROM config WHERE user_id = ?`,
 		`DELETE FROM sessions WHERE user_id = ?`,
 		`DELETE FROM users WHERE id = ?`,