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>

lib/tools.tsx

@@ -1,4 +1,4 @@
-import { ColorIcon, UnitsIcon, ASCIIIcon, MediaIcon, FaviconIcon, QRCodeIcon, AnimateIcon } from '@/components/AppIcons';
+import { ColorIcon, UnitsIcon, ASCIIIcon, MediaIcon, FaviconIcon, QRCodeIcon, AnimateIcon, CalculateIcon } from '@/components/AppIcons';
 
 export interface Tool {
   /** Short display name (e.g. "Color") */
@@ -97,6 +97,17 @@ export const tools: Tool[] = [
     icon: AnimateIcon,
     badges: ['CSS', 'Tailwind v4', '20+ Presets'],
   },
+  {
+    shortTitle: 'Calculate',
+    title: 'Calculator & Grapher',
+    navTitle: 'Calculator',
+    href: '/calculate',
+    description: 'Advanced expression evaluator with interactive function graphing.',
+    summary:
+      'Powerful mathematical calculator powered by Math.js. Evaluate complex expressions, define variables, and plot multiple functions simultaneously on an interactive graph with pan and zoom.',
+    icon: CalculateIcon,
+    badges: ['Math.js', 'Graphing', 'Interactive'],
+  },
 ];
 
 /** Look up a tool by its href path */