Add rulers and cm measurements to personalization canvas

- Display X and Y rulers with 2cm increments
- Show position and size in centimeters (cm) instead of pixels
- Calculate cmPerPx from product print area for accurate conversion
- Rulers positioned above and to the left of design canvas
- Labels show cm measurements for easy reference while personalizing

Users can now see exact placement and dimensions in cm during design,
with visual ruler guides for precise positioning.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/components/DesignCanvas.tsx

@@ -200,6 +200,77 @@ function PrintSurface({
         <div className="pointer-events-none absolute inset-0" style={{ width: DISPLAY, height: DISPLAY }}>
           {photo}
         </div>
+
+        {/* Horizontal Ruler */}
+        <div
+          className="absolute pointer-events-none bg-paper border-b border-line text-xs font-mono"
+          style={{
+            left: stageLeft,
+            top: stageTop - 20,
+            width: stageW,
+            height: 18,
+            display: 'flex',
+            alignItems: 'center',
+            justifyContent: 'space-between',
+            paddingBottom: '2px',
+          }}
+        >
+          {Array.from({ length: Math.ceil((stageW / (10 * (product.printAreaWidthMm / 100)))) }).map((_, i) => {
+            const cm = i * 2;
+            const px = (cm * stageW) / (product.printAreaWidthMm / 10);
+            if (px > stageW) return null;
+            return (
+              <div
+                key={`h-${cm}`}
+                style={{
+                  position: 'absolute',
+                  left: `${px}px`,
+                  fontSize: '9px',
+                  color: '#888',
+                  transform: 'translateX(-50%)',
+                }}
+              >
+                {cm}cm
+              </div>
+            );
+          })}
+        </div>
+
+        {/* Vertical Ruler */}
+        <div
+          className="absolute pointer-events-none bg-paper border-r border-line text-xs font-mono"
+          style={{
+            left: stageLeft - 28,
+            top: stageTop,
+            width: 26,
+            height: stageH,
+            display: 'flex',
+            flexDirection: 'column',
+            alignItems: 'flex-end',
+            paddingRight: '4px',
+          }}
+        >
+          {Array.from({ length: Math.ceil((stageH / (10 * (product.printAreaWidthMm / 100)))) }).map((_, i) => {
+            const cm = i * 2;
+            const px = (cm * stageH) / (product.printAreaWidthMm / 10);
+            if (px > stageH) return null;
+            return (
+              <div
+                key={`v-${cm}`}
+                style={{
+                  position: 'absolute',
+                  top: `${px}px`,
+                  fontSize: '9px',
+                  color: '#888',
+                  transform: 'translateY(-50%)',
+                }}
+              >
+                {cm}
+              </div>
+            );
+          })}
+        </div>
+
         <div className="absolute" style={{ left: stageLeft, top: stageTop, width: stageW, height: stageH }}>
           <Stage
             ref={stageRef}
@@ -348,6 +419,11 @@ export default function DesignCanvas({ product }: { product: ProductDTO }) {
   const activePrintArea = view === 'front' ? frontPrintArea : backPrintArea;
   const activeStageW = activePrintArea.wPct * DISPLAY;
   const activeStageH = activePrintArea.hPct * DISPLAY;
+
+  // Calculate cm per pixel for display
+  const printAreaWidthPx = activePrintArea.wPct * DISPLAY;
+  const cmPerPx = (product.printAreaWidthMm / printAreaWidthPx) / 10;
+  const pxToCm = (px: number) => (px * cmPerPx).toFixed(2);
   const setActiveElements = view === 'front' ? setElementsFront : setElementsBack;
 
   // Keep whichever transformer actually owns the selected element in sync, and
@@ -779,25 +855,25 @@ export default function DesignCanvas({ product }: { product: ProductDTO }) {
 
           {selected && (
             <div className="mt-4 border border-line bg-surface p-4">
-              <p className="mb-3 font-medium text-sm">Position & Size</p>
+              <p className="mb-3 font-medium text-sm">Position & Size (cm)</p>
               <div className="space-y-2 text-xs font-mono">
                 <div className="flex justify-between">
                   <span className="text-muted">X:</span>
-                  <span>{Math.round(selected.x)} px</span>
+                  <span>{pxToCm(selected.x)} cm</span>
                 </div>
                 <div className="flex justify-between">
                   <span className="text-muted">Y:</span>
-                  <span>{Math.round(selected.y)} px</span>
+                  <span>{pxToCm(selected.y)} cm</span>
                 </div>
                 {selected.type === 'image' && (
                   <>
                     <div className="flex justify-between">
                       <span className="text-muted">Width:</span>
-                      <span>{Math.round(selected.width)} px</span>
+                      <span>{pxToCm(selected.width)} cm</span>
                     </div>
                     <div className="flex justify-between">
                       <span className="text-muted">Height:</span>
-                      <span>{Math.round(selected.height)} px</span>
+                      <span>{pxToCm(selected.height)} cm</span>
                     </div>
                   </>
                 )}