feat: add Calculator & Grapher tool

Adds a full-featured mathematical calculator and interactive function
grapher at /calculate. Powered by Math.js v15 with a HiDPI Canvas
renderer for the graph.

- Evaluates arbitrary math expressions (trig, log, complex, matrices,
  factorials, combinatorics, and more) with named variable scope
- Persists history (50 entries) and variables via localStorage
- 32 quick-insert buttons for constants and functions
- Interactive graph: pan (drag), zoom (scroll), crosshair tooltip
  showing cursor coords and f₁(x)…f₈(x) values simultaneously
- Up to 8 color-coded functions with inline color pickers and
  visibility toggles
- Discontinuity detection for functions like tan(x)
- Adaptive grid labels that rescale with zoom
- Responsive layout: 2/5–3/5 split on desktop, tabbed on mobile

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

app/(app)/calculate/page.tsx

@@ -0,0 +1,16 @@
+import type { Metadata } from 'next';
+import Calculator from '@/components/calculate/Calculator';
+import { AppPage } from '@/components/layout/AppPage';
+import { getToolByHref } from '@/lib/tools';
+
+const tool = getToolByHref('/calculate')!;
+
+export const metadata: Metadata = { title: tool.title, description: tool.summary };
+
+export default function CalculatePage() {
+  return (
+    <AppPage title={tool.title} description={tool.summary} icon={tool.icon}>
+      <Calculator />
+    </AppPage>
+  );
+}