make PDP variant selection work without JS

Variant options (colour, size) are now URL params handled via
handle_params instead of phx-click events. Swatches and size buttons
render as patch links in shop mode, so changing variants works as
plain navigation without JS. Quantity is now a number input that
submits with the form. Unavailable variants render as disabled spans.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

assets/css/shop/components.css

@@ -578,11 +578,18 @@
     position: relative;
     cursor: pointer;
     padding: 0;
+    display: inline-block;
+    text-decoration: none;
 
     &[aria-pressed="true"] {
       border-color: var(--t-accent);
       --tw-ring-color: var(--t-accent);
     }
+
+    &[aria-disabled="true"] {
+      opacity: 0.3;
+      cursor: not-allowed;
+    }
   }
 
   .size-btn {
@@ -594,11 +601,18 @@
     font-weight: 500;
     transition: all 0.2s ease;
     cursor: pointer;
+    text-decoration: none;
 
     &[aria-pressed="true"] {
       border-color: var(--t-accent);
       background: color-mix(in oklch, var(--t-accent) 10%, transparent);
     }
+
+    &[aria-disabled="true"] {
+      opacity: 0.3;
+      cursor: not-allowed;
+      border-style: dashed;
+    }
   }
 
   /* ── Quantity selector ── */
@@ -643,9 +657,20 @@
     color: var(--t-text-primary);
     padding: 0.5rem 1rem;
     border-inline: 2px solid var(--t-border-default);
-    min-width: 3rem;
+    width: 3.5rem;
     text-align: center;
     font-variant-numeric: tabular-nums;
+    background: none;
+    border-top: none;
+    border-bottom: none;
+    font: inherit;
+    appearance: textfield;
+
+    &::-webkit-inner-spin-button,
+    &::-webkit-outer-spin-button {
+      appearance: none;
+      margin: 0;
+    }
   }
 
   .stock-in {