feat: add theme palettes (#1326)

Add Neutral, Indigo, Forest, and Sand palettes independently of Light, Dark, and System. Persist and hydrate the selection, add the accessible settings picker, and include the validated review fixes for keyboard navigation and Swedish copy.

lib/theme/__tests__/palettes.test.ts

@@ -0,0 +1,25 @@
+import { describe, expect, it } from 'vitest'
+import {
+  DEFAULT_PALETTE,
+  PALETTE_STORAGE_KEY,
+  PALETTE_VALUES,
+  isPalette,
+} from '@/lib/theme/palettes'
+
+describe('palettes', () => {
+  it('keeps neutral as the backward-compatible default', () => {
+    expect(DEFAULT_PALETTE).toBe('neutral')
+    expect(PALETTE_STORAGE_KEY).toBe('accounted-palette')
+  })
+
+  it.each(PALETTE_VALUES)('accepts the %s palette', (palette) => {
+    expect(isPalette(palette)).toBe(true)
+  })
+
+  it.each([null, undefined, '', 'light', 'dark', 'sepia', 1])(
+    'rejects unsupported palette value %s',
+    (value) => {
+      expect(isPalette(value)).toBe(false)
+    },
+  )
+})