Add favorite toggle to yoga poses

Exercises and workouts both have a favorite toggle; yoga poses had an
otherwise identical browse/filter UX but no way to favorite them - an
inconsistency rather than a deliberate omission. Mirrors the existing
exercise/workout favorite pattern exactly: a pivot table, a
belongsToMany on User, and a toggle button on the pose detail page.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/Models/User.php

@@ -79,6 +79,11 @@ class User extends Authenticatable
         return $this->belongsToMany(Workout::class, 'workout_favorites')->withTimestamps();
     }
 
+    public function favoriteYogaPoses()
+    {
+        return $this->belongsToMany(YogaPose::class, 'yoga_pose_favorites')->withTimestamps();
+    }
+
     // Helpers
     public function hasFavoritedExercise(Exercise $exercise): bool
     {
@@ -89,4 +94,9 @@ class User extends Authenticatable
     {
         return $this->favoriteWorkouts()->where('workout_id', $workout->id)->exists();
     }
+
+    public function hasFavoritedYogaPose(YogaPose $pose): bool
+    {
+        return $this->favoriteYogaPoses()->where('yoga_pose_id', $pose->id)->exists();
+    }
 }