feat(01-input-ux-01): create StepperInput controlled component

- Reusable stepper with +/- buttons flanking a number input
- Handles min clamping, max constraint, decimal steps
- Controlled component (no internal state): value/onChange props
- 44px touch targets, 16px font, aria-labels present
- Rejects non-numeric input silently

frontend/src/components/StepperInput.jsx

@@ -0,0 +1,89 @@
+function StepperInput({
+  value = '',
+  onChange,
+  step = 1,
+  min = 0,
+  max = null,
+  label = 'Value',
+  suffix = '',
+  disabled = false,
+}) {
+  const numValue = parseFloat(value) || 0;
+
+  function handleInputChange(e) {
+    const raw = e.target.value;
+    if (raw === '') {
+      onChange('');
+      return;
+    }
+    const parsed = parseFloat(raw);
+    if (isNaN(parsed)) return;
+    if (parsed < min) {
+      onChange(String(min));
+    } else if (max !== null && parsed > max) {
+      onChange(String(max));
+    } else {
+      onChange(String(parsed));
+    }
+  }
+
+  function handleDecrement() {
+    if (disabled) return;
+    const newVal = Math.max(min, numValue - step);
+    onChange(String(newVal));
+  }
+
+  function handleIncrement() {
+    if (disabled) return;
+    const newVal = numValue + step;
+    if (max === null || newVal <= max) {
+      onChange(String(newVal));
+    }
+  }
+
+  const canDecrement = numValue > min;
+  const canIncrement = max === null || numValue < max;
+
+  return (
+    <div className="stepper-wrapper" role="group" aria-labelledby={`stepper-label-${label}`}>
+      <label id={`stepper-label-${label}`} className="stepper-label">{label}</label>
+      <div className="stepper-container">
+        <button
+          type="button"
+          className="stepper-btn stepper-minus"
+          onClick={handleDecrement}
+          disabled={!canDecrement || disabled}
+          aria-label={`Decrease ${label}`}
+        >
+          −
+        </button>
+        <div className="stepper-input-wrapper">
+          <input
+            type="number"
+            value={value}
+            onChange={handleInputChange}
+            min={min}
+            max={max ?? undefined}
+            step={step}
+            inputMode={step % 1 === 0 ? 'numeric' : 'decimal'}
+            className="stepper-input"
+            aria-label={label}
+            disabled={disabled}
+          />
+          {suffix && <span className="input-suffix">{suffix}</span>}
+        </div>
+        <button
+          type="button"
+          className="stepper-btn stepper-plus"
+          onClick={handleIncrement}
+          disabled={!canIncrement || disabled}
+          aria-label={`Increase ${label}`}
+        >
+          +
+        </button>
+      </div>
+    </div>
+  );
+}
+
+export default StepperInput;