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/calculate/store.ts

@@ -0,0 +1,107 @@
+import { create } from 'zustand';
+import { persist } from 'zustand/middleware';
+
+export const FUNCTION_COLORS = [
+  '#f472b6',
+  '#60a5fa',
+  '#4ade80',
+  '#fb923c',
+  '#a78bfa',
+  '#22d3ee',
+  '#fbbf24',
+  '#f87171',
+];
+
+export interface HistoryEntry {
+  id: string;
+  expression: string;
+  result: string;
+  error: boolean;
+}
+
+export interface GraphFunction {
+  id: string;
+  expression: string;
+  color: string;
+  visible: boolean;
+}
+
+interface CalculateStore {
+  expression: string;
+  history: HistoryEntry[];
+  variables: Record<string, string>;
+  graphFunctions: GraphFunction[];
+  setExpression: (expr: string) => void;
+  addToHistory: (entry: Omit<HistoryEntry, 'id'>) => void;
+  clearHistory: () => void;
+  setVariable: (name: string, value: string) => void;
+  removeVariable: (name: string) => void;
+  addGraphFunction: () => void;
+  updateGraphFunction: (
+    id: string,
+    updates: Partial<Pick<GraphFunction, 'expression' | 'color' | 'visible'>>
+  ) => void;
+  removeGraphFunction: (id: string) => void;
+}
+
+const uid = () => `${Date.now()}-${Math.random().toString(36).slice(2, 7)}`;
+
+export const useCalculateStore = create<CalculateStore>()(
+  persist(
+    (set) => ({
+      expression: '',
+      history: [],
+      variables: {},
+      graphFunctions: [
+        { id: 'init-1', expression: 'sin(x)', color: FUNCTION_COLORS[0], visible: true },
+        { id: 'init-2', expression: 'cos(x)', color: FUNCTION_COLORS[1], visible: true },
+      ],
+
+      setExpression: (expression) => set({ expression }),
+
+      addToHistory: (entry) =>
+        set((state) => ({
+          history: [{ ...entry, id: uid() }, ...state.history].slice(0, 50),
+        })),
+
+      clearHistory: () => set({ history: [] }),
+
+      setVariable: (name, value) =>
+        set((state) => ({ variables: { ...state.variables, [name]: value } })),
+
+      removeVariable: (name) =>
+        set((state) => {
+          const v = { ...state.variables };
+          delete v[name];
+          return { variables: v };
+        }),
+
+      addGraphFunction: () =>
+        set((state) => {
+          const used = new Set(state.graphFunctions.map((f) => f.color));
+          const color =
+            FUNCTION_COLORS.find((c) => !used.has(c)) ??
+            FUNCTION_COLORS[state.graphFunctions.length % FUNCTION_COLORS.length];
+          return {
+            graphFunctions: [
+              ...state.graphFunctions,
+              { id: uid(), expression: '', color, visible: true },
+            ],
+          };
+        }),
+
+      updateGraphFunction: (id, updates) =>
+        set((state) => ({
+          graphFunctions: state.graphFunctions.map((f) =>
+            f.id === id ? { ...f, ...updates } : f
+          ),
+        })),
+
+      removeGraphFunction: (id) =>
+        set((state) => ({
+          graphFunctions: state.graphFunctions.filter((f) => f.id !== id),
+        })),
+    }),
+    { name: 'kit-calculate-v1' }
+  )
+);