feat: best scale auto-choose

script.js

@@ -127,9 +127,27 @@ addEventListener('DOMContentLoaded', () => {
   })
 
   { // Choose best scale for screen height
-    const height = document.documentElement.clientHeight;
-    const padding = getComputedStyle(document.body).paddingTop;
-    // TODO
+    let padding = getComputedStyle(document.body).paddingBottom
+    // remove `px` at the end and convert to number
+    padding = Number(padding.slice(0, padding.length - 2))
+
+    // compute available height
+    const height = 
+      document.documentElement.clientHeight
+        - canvas.offsetTop // substract height of everything above canvas
+        - padding // substract body padding
+
+    const width =
+      document.documentElement.clientWidth
+        - padding * 2
+
+    options.scale.value = Math.max(
+      intDiv(
+        width < height ? width : height,
+        25, // divide least dimension by QR version 2 size
+      ),
+      10, // minimum scale for this auto-detect algorithm
+    )
   }
 
   { // QR version related stuff