feat(01-01): implement palette.ts as single source of truth for category colors

- Export CategoryType union type with 7 category strings
- Export CategoryShades interface with light/medium/base fields
- Export palette Record with oklch values matching --chart-* CSS tokens
- Export headerGradient() returning CSSProperties with linear-gradient
- Export overviewHeaderGradient() with multi-stop sky/lavender/green gradient
- Export amountColorClass() for text-success/text-warning/text-destructive
- All 20 unit tests pass

frontend/src/lib/palette.ts

@@ -0,0 +1,102 @@
+import type React from 'react'
+
+export type CategoryType =
+  | 'income'
+  | 'bill'
+  | 'variable_expense'
+  | 'debt'
+  | 'saving'
+  | 'investment'
+  | 'carryover'
+
+export interface CategoryShades {
+  light: string
+  medium: string
+  base: string
+}
+
+export const palette: Record<CategoryType, CategoryShades> = {
+  income: {
+    light: 'oklch(0.96 0.04 145)',
+    medium: 'oklch(0.88 0.08 145)',
+    base: 'oklch(0.76 0.14 145)',
+  },
+  bill: {
+    light: 'oklch(0.96 0.03 250)',
+    medium: 'oklch(0.88 0.07 250)',
+    base: 'oklch(0.76 0.12 250)',
+  },
+  variable_expense: {
+    light: 'oklch(0.97 0.04 85)',
+    medium: 'oklch(0.90 0.08 85)',
+    base: 'oklch(0.80 0.14 85)',
+  },
+  debt: {
+    light: 'oklch(0.96 0.04 15)',
+    medium: 'oklch(0.88 0.08 15)',
+    base: 'oklch(0.76 0.13 15)',
+  },
+  saving: {
+    light: 'oklch(0.95 0.04 280)',
+    medium: 'oklch(0.87 0.08 280)',
+    base: 'oklch(0.75 0.13 280)',
+  },
+  investment: {
+    light: 'oklch(0.96 0.04 320)',
+    medium: 'oklch(0.88 0.07 320)',
+    base: 'oklch(0.76 0.12 320)',
+  },
+  carryover: {
+    light: 'oklch(0.96 0.03 210)',
+    medium: 'oklch(0.88 0.06 210)',
+    base: 'oklch(0.76 0.11 210)',
+  },
+}
+
+/**
+ * Returns a CSSProperties object with a linear-gradient background
+ * using the light and medium shades of the given category type.
+ */
+export function headerGradient(type: CategoryType): React.CSSProperties {
+  const shades = palette[type]
+  return {
+    background: `linear-gradient(to right, ${shades.light}, ${shades.medium})`,
+  }
+}
+
+/**
+ * Returns a CSSProperties object with a multi-stop linear-gradient
+ * for the FinancialOverview header (sky via lavender to green).
+ */
+export function overviewHeaderGradient(): React.CSSProperties {
+  return {
+    background: `linear-gradient(to right, ${palette.carryover.light}, ${palette.saving.light}, ${palette.income.light})`,
+  }
+}
+
+export interface AmountColorClassOpts {
+  type: CategoryType
+  actual: number
+  budgeted: number
+  isIncome?: boolean
+  isAvailable?: boolean
+}
+
+/**
+ * Returns a Tailwind utility class for coloring a monetary amount:
+ * - isIncome or isAvailable: positive → 'text-success', negative → 'text-destructive', zero → ''
+ * - Expense: actual > budgeted → 'text-warning', else → ''
+ */
+export function amountColorClass(opts: AmountColorClassOpts): string {
+  const { actual, budgeted, isIncome, isAvailable } = opts
+
+  if (isIncome || isAvailable) {
+    if (actual > 0) return 'text-success'
+    if (actual < 0) return 'text-destructive'
+    return ''
+  }
+
+  // Expense path
+  if (actual > budgeted) return 'text-warning'
+  return ''
+}