UX: fraction buttons for pz unit in use form + fix qty display

- _pzFractionLabel(): formats 2.5 → '2½', 2.25 → '2¼', etc.
- formatQuantity/formatQuantityParts: use fraction label for pz (was rounding to int)
- loadUseInventoryInfo normal mode: show ¼/½/¾/1 intero quick-select buttons for pz
- setPzFraction(): sets quantity input + syncs active button highlight
- adjustUseQty pz: step = 0.5 (half-piece) instead of 1; syncs frac button on ±
- CSS: .pz-fraction-btns, .fraction-btn-row, .frac-btn, .frac-btn.active

assets/css/style.css

@@ -963,6 +963,40 @@ body {
     color: var(--text-light);
 }
 
+/* Fraction quick-select for pz unit */
+.pz-fraction-btns {
+    width: 100%;
+}
+.pz-fraction-btns .form-hint {
+    font-size: 0.82rem;
+    color: var(--text-light);
+    text-align: center;
+    margin: 0 0 6px;
+}
+.fraction-btn-row {
+    display: flex;
+    gap: 6px;
+}
+.frac-btn {
+    flex: 1;
+    padding: 8px 4px;
+    border: 2px solid var(--primary);
+    background: transparent;
+    color: var(--primary);
+    border-radius: var(--radius-sm);
+    font-size: 0.88rem;
+    font-weight: 600;
+    cursor: pointer;
+    transition: background 0.15s, color 0.15s;
+}
+.frac-btn.active {
+    background: var(--primary);
+    color: #fff;
+}
+.frac-btn:hover:not(.active) {
+    background: var(--primary-light, rgba(74,158,255,0.12));
+}
+
 /* ===== SCANNER ===== */
 .scan-container {
     display: flex;