feat(engagement): localize limits and streaks

Use reported user timezones for daily response limits, expose couple streak state and reminders, and improve swipe and live-drawing synchronization with focused tests and documentation.

apps/mobile/src/lib/profileSettingsApi.ts

@@ -13,6 +13,7 @@ export interface ProfileUpdate {
     onboarding_version?: number;
     public_key_jwk?: Record<string, unknown> | null;
     push_token?: string | null;
+    timezone?: string | null;
 }
 
 type PreferenceKey = keyof Omit<NotificationPreferences, 'user_id' | 'created_at' | 'updated_at'>;

apps/mobile/src/lib/reportedTimezone.ts

@@ -0,0 +1,68 @@
+import * as SecureStore from "expo-secure-store";
+import { Platform } from "react-native";
+
+import { profileSettingsApi } from "./profileSettingsApi";
+
+const STORAGE_KEY_PREFIX = "reported_timezone";
+
+const buildKey = (userId: string): string => `${STORAGE_KEY_PREFIX}_${userId}`;
+
+/** The IANA zone this device is in, or null when the platform can't resolve one. */
+export function resolveDeviceTimezone(): string | null {
+    try {
+        const zone = Intl.DateTimeFormat().resolvedOptions().timeZone;
+        return zone && zone.length <= 64 ? zone : null;
+    } catch {
+        return null;
+    }
+}
+
+async function readStored(key: string): Promise<string | null> {
+    try {
+        if (Platform.OS === "web") {
+            if (typeof window !== "undefined" && typeof window.localStorage !== "undefined") {
+                return window.localStorage.getItem(key);
+            }
+            return null;
+        }
+        return await SecureStore.getItemAsync(key);
+    } catch (error) {
+        console.error("Error reading reported timezone:", error);
+        return null;
+    }
+}
+
+async function writeStored(key: string, zone: string): Promise<void> {
+    try {
+        if (Platform.OS === "web") {
+            if (typeof window !== "undefined" && typeof window.localStorage !== "undefined") {
+                window.localStorage.setItem(key, zone);
+            }
+            return;
+        }
+        await SecureStore.setItemAsync(key, zone);
+    } catch (error) {
+        console.error("Error saving reported timezone:", error);
+    }
+}
+
+/**
+ * Report the device timezone to the server so the daily response limit buckets on the
+ * user's local day. Cached locally, so this costs one write on install and one per
+ * actual travel event rather than one per foreground.
+ */
+export async function syncTimezone(userId: string): Promise<void> {
+    const zone = resolveDeviceTimezone();
+    if (!zone) return;
+
+    const key = buildKey(userId);
+    if (await readStored(key) === zone) return;
+
+    try {
+        await profileSettingsApi.updateProfile({ timezone: zone });
+        await writeStored(key, zone);
+    } catch (error) {
+        // Non-critical: the server falls back to UTC until this succeeds.
+        console.error("Failed to sync timezone:", error);
+    }
+}