diff --git a/src/lib/settings/store.ts b/src/lib/settings/store.ts
index d3740569..0a931e50 100644
--- a/src/lib/settings/store.ts
+++ b/src/lib/settings/store.ts
@@ -1,20 +1,84 @@
 'use client';
 
+/**
+ * @module settings/store
+ *
+ * Zustand store for user application settings.
+ *
+ * ## Quick start
+ *
+ * ```tsx
+ * import { useSettingsStore } from '@/lib/settings/store';
+ *
+ * function MyComponent() {
+ *   const theme = useSettingsStore((s) => s.settings.theme);
+ *   const patchSettings = useSettingsStore((s) => s.patchSettings);
+ *
+ *   return (
+ *     <button onClick={() => patchSettings({ theme: 'dark' })}>
+ *       Switch to dark mode
+ *     </button>
+ *   );
+ * }
+ * ```
+ *
+ * ## Persistence
+ *
+ * Settings are automatically persisted to `localStorage` under the key defined by
+ * `SETTINGS_STORAGE_KEY`. The store handles SSR gracefully by falling back to a no-op
+ * storage when `window` is unavailable.
+ *
+ * ## Sync
+ *
+ * Use `fetchRemoteSettings` / `pushRemoteSettings` from `@/lib/settings/sync` to
+ * synchronise settings with the server. `updatedAt` acts as a vector clock: the
+ * side with the larger value wins.
+ *
+ * ## Export / Import
+ *
+ * Use `buildExportEnvelope` / `parseExportedSettings` from `@/lib/settings/export-import`
+ * to serialise settings to a portable JSON file that users can download and re-import.
+ */
 import { create } from 'zustand';
 import { persist, createJSONStorage } from 'zustand/middleware';
 import { SETTINGS_SCHEMA_VERSION, SETTINGS_STORAGE_KEY } from './constants';
 import { type AppSettings, appSettingsSchema, createDefaultSettings } from './types';
 
 interface SettingsStoreActions {
+  /**
+   * Merge a partial update into the current settings.
+   * Validates the merged result against `appSettingsSchema`; silently ignores invalid patches.
+   * `language` is trimmed and clamped to 24 chars; empty strings fall back to `'en'`.
+   * Automatically updates `updatedAt` to `Date.now()`.
+   */
   patchSettings: (partial: Partial<AppSettings>) => void;
+
+  /**
+   * Overwrite all settings with a validated `AppSettings` object (e.g. after a remote sync or import).
+   * Pass `markSynced = true` to set `lastSyncedAt` to `updatedAt` in the same write.
+   * Invalid settings objects are silently dropped.
+   */
   replaceSettings: (settings: AppSettings, updatedAt: number, markSynced?: boolean) => void;
+
+  /**
+   * Restore all settings to the application defaults (see `createDefaultSettings`).
+   * Resets `updatedAt` to `Date.now()` and clears `lastSyncedAt`.
+   */
   resetSettings: () => void;
+
+  /**
+   * Record the timestamp of the last successful remote sync.
+   * Pass `null` to clear the sync marker (e.g. after a reset).
+   */
   setLastSyncedAt: (t: number | null) => void;
 }
 
 interface SettingsSlice extends SettingsStoreActions {
+  /** Current user preferences. Always conforms to `appSettingsSchema`. */
   settings: AppSettings;
+  /** Unix ms timestamp of the last local settings mutation. Used as a vector clock for sync. */
   updatedAt: number;
+  /** Unix ms timestamp of the most recent successful remote sync, or `null` if never synced. */
   lastSyncedAt: number | null;
 }
 
diff --git a/src/lib/settings/types.ts b/src/lib/settings/types.ts
index 28e6c7d9..8c724dbf 100644
--- a/src/lib/settings/types.ts
+++ b/src/lib/settings/types.ts
@@ -1,9 +1,22 @@
 import { z } from 'zod';
 import { SETTINGS_SCHEMA_VERSION } from './constants';
 
+/** User-selectable colour scheme. `'system'` follows the OS preference. */
 export const themePreferenceSchema = z.enum(['light', 'dark', 'system']);
 export type ThemePreference = z.infer<typeof themePreferenceSchema>;
 
+/**
+ * Validated schema for all user-configurable application settings.
+ *
+ * Fields:
+ * - `version`              — Schema version; bumped when new fields are added (see `SETTINGS_SCHEMA_VERSION`).
+ * - `theme`                — Colour scheme: `'light'`, `'dark'`, or `'system'` (follows OS preference).
+ * - `language`             — BCP-47 locale tag (e.g. `'en'`, `'fr-CA'`), max 24 chars; defaults to `navigator.language`.
+ * - `notificationsEnabled` — Master toggle for in-app push/toast notifications.
+ * - `emailNotifications`   — Whether transactional and digest emails should be sent.
+ * - `prefetchingEnabled`   — Pre-fetches linked pages on hover for faster navigation; disable on slow connections.
+ * - `reducedMotion`        — Suppresses non-essential animations for users who prefer reduced motion.
+ */
 export const appSettingsSchema = z.object({
   version: z.literal(SETTINGS_SCHEMA_VERSION),
   theme: themePreferenceSchema,
@@ -14,18 +27,30 @@ export const appSettingsSchema = z.object({
   reducedMotion: z.boolean(),
 });
 
+/** Fully typed representation of all user settings. Inferred from `appSettingsSchema`. */
 export type AppSettings = z.infer<typeof appSettingsSchema>;
 
+/**
+ * Shape persisted by the Zustand store to `localStorage` (key: `SETTINGS_STORAGE_KEY`).
+ * Includes `updatedAt` and `lastSyncedAt` for conflict resolution during remote sync.
+ */
 export const settingsStoreStateSchema = z.object({
   settings: appSettingsSchema,
+  /** Unix ms timestamp of the last local mutation. Used as a vector clock for sync. */
   updatedAt: z.number(),
+  /** Unix ms timestamp of the last successful remote sync, or `null` if never synced. */
   lastSyncedAt: z.number().nullable(),
 });
 
 export type SettingsStorePersistedShape = z.infer<typeof settingsStoreStateSchema>;
 
+/**
+ * JSON envelope produced by `buildExportEnvelope` and consumed by `parseExportedSettings`.
+ * Including `exportedAt` and `version` allows future migrations to detect stale exports.
+ */
 export const exportedSettingsEnvelopeSchema = z.object({
   version: z.literal(SETTINGS_SCHEMA_VERSION),
+  /** ISO-8601 UTC timestamp of when the file was exported. */
   exportedAt: z.string(),
   settings: appSettingsSchema,
   updatedAt: z.number(),
@@ -33,6 +58,13 @@ export const exportedSettingsEnvelopeSchema = z.object({
 
 export type ExportedSettingsEnvelope = z.infer<typeof exportedSettingsEnvelopeSchema>;
 
+/**
+ * Returns an `AppSettings` object with safe, well-defined defaults.
+ *
+ * - `theme` defaults to `'system'` so the OS preference is respected out of the box.
+ * - `language` is read from `navigator.language` when available, falling back to `'en'`.
+ * - All notification and UX toggles default to their most permissive value.
+ */
 export function createDefaultSettings(): AppSettings {
   return {
     version: SETTINGS_SCHEMA_VERSION,