Calculator v1.6: round-screen keypad with arithmetic

- 19-key layout fitted to round display, centered focus, scrollable list
- full arithmetic: +-*/ %, decimals, +/- , AC, divide-by-zero, chained ops
- scroll-to-center focus with top/bottom spacers, high-contrast keys
- tests: arithmetic + geometry/contrast

.gitignore

@@ -0,0 +1,5 @@
+node_modules/
+build/
+dist/
+sign/*.pem
+