Sign-in loads the identity it already has

🔴 **This is why signing out and back in broke everything.** Sign-in only
ever consulted the in-memory `identity` field, and sign-out clears it —
so it minted a fresh `DeviceIdentity` and OVERWROTE the vault's, making
this phone a brand new device on every single sign-in.

Everything the owner hit follows from that one line:

  - contacts saw a safety-number change and had to accept new keys;
  - the account accumulated a registered device per sign-in, and fan-out
    is per device, so half of everything sent to this account went to a
    mailbox nobody reads — which is why the failures looked random and
    different each time;
  - the relay's public key log had not caught up with a key registered
    seconds earlier, so even after accepting, the next send failed on
    "the keys offered by this contact are not in the public log".

⚠️ It was harmless until it wasn't. Sign-out used to destroy the vault,
so there was never a stored identity to find and minting one was
correct. Keeping the device across a plain sign-out — which is what
makes signing out cheap — is what turned this into the bug it always
looked like. The two changes had to land together and only one did.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/main/kotlin/io/flamenet/messenger/app/MessengerViewModel.kt

@@ -506,10 +506,26 @@ class MessengerViewModel(app: Application) : AndroidViewModel(app) {
         }
 
         val vault = vault ?: error("the vault is not open")
-        val device = identity ?: DeviceIdentity.create().also {
-            vault.saveIdentity(it)
-            identity = it
-        }
+        // 🔴 **The stored identity FIRST, and this line is why signing out and
+        // back in used to cost every contact a prompt.** Sign-in only ever
+        // consulted the in-memory field, and sign-out clears it — so it minted
+        // a fresh identity and overwrote the vault's, making this phone a brand
+        // new device on every sign-in. Contacts saw a safety-number change,
+        // sends were refused until they accepted it, and the relay's public key
+        // log had not yet caught up with the key being offered, so even after
+        // accepting, the next send failed on "the keys offered by this contact
+        // are not in the public log".
+        //
+        // ⚠️ It was harmless right up until it wasn't: sign-out used to destroy
+        // the vault, so there was never a stored identity to find and minting
+        // one was correct. Keeping the device across a plain sign-out is what
+        // turned this into the bug it always looked like.
+        val device = identity
+            ?: vault.loadIdentity()?.also { identity = it }
+            ?: DeviceIdentity.create().also {
+                vault.saveIdentity(it)
+                identity = it
+            }
 
         val relay = RelayClient(config.baseUrl, username = account.username, appPassword = credential)
         relay.useRelayAuth(device)