refactor: consolidate API-key crypto into src/lib/crypto with compat tests

Encrypt (Options) and decrypt (background) now share one module; wire
format unchanged. Tests include fixtures proving values encrypted by the
old inline Options code still decrypt, plus tamper/wrong-key cases. The
module documents the honest threat model (key co-located with ciphertext
= obfuscation, not encryption).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

entrypoints/options/Options.tsx

@@ -1,7 +1,7 @@
 import React, { useEffect, useRef, useState } from 'react';
 import { createRoot } from 'react-dom/client';
-import nacl from 'tweetnacl';
 import { safeSendMessage, safeStorageGet, safeStorageSet } from '@lib/messaging';
+import { encryptApiKey, getOrCreateEncKey } from '@lib/crypto';
 
 // ─── Provider config ──────────────────────────────────────────────────────────
 
@@ -33,23 +33,6 @@ const PROVIDERS = [
   },
 ];
 
-// ─── Encryption helpers ───────────────────────────────────────────────────────
-
-async function getOrCreateEncKey(): Promise<Uint8Array> {
-  return new Promise((resolve) => {
-    chrome.storage.local.get(['encKey'], (result) => {
-      if (result.encKey) {
-        resolve(Uint8Array.from(atob(result.encKey as string), c => c.charCodeAt(0)));
-      } else {
-        const key = nacl.randomBytes(32);
-        const keyB64 = btoa(String.fromCharCode(...key));
-        chrome.storage.local.set({ encKey: keyB64 });
-        resolve(key);
-      }
-    });
-  });
-}
-
 // ─── Styles ───────────────────────────────────────────────────────────────────
 
 const styles = {
@@ -275,13 +258,7 @@ function OptionsPage() {
 
     try {
       const key = await getOrCreateEncKey();
-      const nonce = nacl.randomBytes(24);
-      const encoded = new TextEncoder().encode(apiKey.trim());
-      const encrypted = nacl.secretbox(encoded, nonce, key);
-      const combined = new Uint8Array(nonce.length + encrypted.length);
-      combined.set(nonce);
-      combined.set(encrypted, nonce.length);
-      const apiKeyEncB64 = btoa(String.fromCharCode(...combined));
+      const apiKeyEncB64 = encryptApiKey(apiKey.trim(), key);
 
       safeStorageSet({ apiKeyEnc: apiKeyEncB64, provider, model }, () => {
         if (chrome.runtime.lastError) {