feat(01-02): integrate WeightInput and RepsInput into ExerciseCard set rows

- Import WeightInput and RepsInput in WorkoutPage.jsx
- Replace bare <input type="number"> elements with stepper components
- Update .set-inputs alignment to flex-start for taller steppers
- Update .set-row alignment to flex-start
- Remove now-redundant .weight-input and .reps-input CSS rules (main + mobile)

frontend/src/App.css

@@ -281,7 +281,7 @@
 
 .set-row {
   display: flex;
-  align-items: center;
+  align-items: flex-start;
   gap: 0.75rem;
   padding: 0.75rem;
   background: var(--bg-secondary);
@@ -303,25 +303,8 @@
 .set-inputs {
   flex: 1;
   display: flex;
-  align-items: center;
-  gap: 0.5rem;
-}
-
-.weight-input,
-.reps-input {
-  width: 70px;
-  padding: 0.6rem;
-  background: var(--bg-card);
-  border: 1px solid var(--border);
-  border-radius: 6px;
-  color: var(--text-primary);
-  font-size: 1rem;
-  text-align: center;
-}
-
-.weight-input:focus,
-.reps-input:focus {
-  border-color: var(--accent);
+  align-items: flex-start;
+  gap: 0.75rem;
 }
 
 .input-separator {
@@ -362,11 +345,6 @@
     padding: 0.75rem;
   }
   
-  .weight-input,
-  .reps-input {
-    width: 60px;
-    padding: 0.5rem;
-  }
 }
 
 /* Safe area for notched phones */