Add remember-me and OTP autofill, fix session persistence

Sign-in gains a "Keep me signed in" checkbox: checked issues a 30-day
token and prefills the address next launch, unchecked drops the session
to 12 hours and forgets the address. The TTL is chosen server-side in
the login route.

Emailed codes are now reachable without retyping. OtpField opts into the
iOS one-time-code keyboard suggestion and raises a paste chip when the
user returns from Gmail with a code on the clipboard. The mails put the
code first in the subject and body, which is what makes Gmail render its
"Copy code" notification action at all.

Fixes found along the way:

- Session was wiped on every launch. decodeJwtExp used atob, which
  neither RN 0.74 nor Expo SDK 51 defines, so it threw, returned null,
  and the caller read that as "expired" and deleted the token. Replaced
  with a dependency-free base64url decoder, and restore now only
  discards a session it can prove is expired.
- Verification and reset codes counted attempts but never enforced them,
  leaving a 6-digit code open to unlimited guessing. Both routes now
  charge the attempt before comparing so concurrent guesses can't race
  past the cap of five, and compare in constant time.
- A wrong verification code showed the "Verified" success screen:
  onModalHide fired unconditionally, so the failure state advanced the
  flow. Only an explicit "verified" state does that now.
- fetchAPI discarded the server's error body, so the UI substring-matched
  synthetic status strings and showed "Could not sign in" for everything.
  It now throws ApiError carrying status and the server's message.
- Login answered a missing account faster than a wrong password; it now
  runs the same scrypt work either way.
- Blank email or password is caught client-side instead of surfacing as
  an opaque 400, and a failed attempt only clears the password on a 401.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

lib/fetch.ts

@@ -12,6 +12,17 @@ export const clearAuthToken = () => {
   authToken = null;
 };
 
+/** Carries the HTTP status so callers can branch on it instead of on text. */
+export class ApiError extends Error {
+  status: number;
+
+  constructor(status: number, message: string) {
+    super(message);
+    this.name = "ApiError";
+    this.status = status;
+  }
+}
+
 export const fetchAPI = async (url: string, options?: RequestInit) => {
   try {
     const headers = new Headers(options?.headers);
@@ -20,12 +31,21 @@ export const fetchAPI = async (url: string, options?: RequestInit) => {
     }
 
     const response = await fetch(url, { ...options, headers });
+
     if (!response.ok) {
-      throw new Error(`HTTP error! status: ${response.status}`);
+      // Every route answers with `{ error }`; keep that text so the UI can show
+      // what actually went wrong instead of guessing from a status code.
+      const body = await response.json().catch(() => null);
+
+      throw new ApiError(
+        response.status,
+        body?.error ?? `Request failed with status ${response.status}.`,
+      );
     }
+
     return await response.json();
   } catch (error) {
-    console.error("Fetch error:", error);
+    console.error(`Fetch error: ${url}`, error);
     throw error;
   }
 };