fix: sync identity stores and append signatures to outgoing emails (#15)

- Add syncIdentities() to auth store to propagate identity changes from
  identity store, fixing stale data that caused save failures and duplicates
- Call syncIdentities() after every create, update, and delete in the
  identity manager modal
- Switch email composer to read identities from identity store for
  consistency with the rest of the app
- Append identity text signature (with RFC 3676 separator) to email body
  when sending from the composer and quick reply paths
- Add tests for syncIdentities and signature appending logic

app/[locale]/page.tsx

@@ -761,12 +761,18 @@ export default function Home() {
 
     const primaryIdentity = identities[0];
 
+    // Append signature from the primary identity
+    let finalBody = body;
+    if (primaryIdentity?.textSignature) {
+      finalBody = body + '\n\n-- \n' + primaryIdentity.textSignature;
+    }
+
     // Send reply with just the body text
     await sendEmail(
       client,
       [sender.email],
       `Re: ${selectedEmail.subject || "(no subject)"}`,
-      body,
+      finalBody,
       undefined,
       undefined,
       primaryIdentity?.id,