core: refuse an account index BIP-32 cannot harden, whichever way it arrives

The counter path checked it, the explicit one did not, and the parser is not
the only caller: processChatCommand takes APIWalletBind from library callers
too. hardened leaves an index at or above 2^31 alone, so account i and
i + 2^31 are the same key while the duplicate check compares the stored
integers, and two profiles could hold one account's names.

The rfc now says what hiding a profile from /_wallet does and does not
protect: the seed is one per device, so whoever unlocks any profile can
derive every account.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Rvc3HbiWBTqbAvRT45G5oX

docs/rfcs/2026-09-10-name-ownership-keys.md

@@ -124,6 +124,18 @@ import, so `/_wallet bind` with no account can hand out one that already owns
 names. Only a scan of owned names can restore the mark, and that lands with the
 registrar.
 
+## Hidden profiles
+
+`/_wallet` names the other profiles on the seed and never numbers them, so a
+hidden profile leaves no gap in a list of account indexes.
+
+That hides its existence from the listing, and nothing more. The seed is one per
+device, so whoever unlocks any profile can export the phrase and derive every
+account, including a hidden profile's. A hidden profile's names are not
+pseudonymous against someone who already holds the device and one password. This
+is a consequence of one seed per device, and a key per profile rather than per
+device is what would change it.
+
 ## Scope
 
 Not here, and unchanged from the prototype: buying a name, the names protocol,

src/Simplex/Chat/Store/Wallets.hs

@@ -88,20 +88,22 @@ bindAccountIndex db user@User {userId} sId = \case
         if acct >= 0x80000000
           then pure $ Left "no free account on this key"
           else Right () <$ bindUser db userId sId acct
-  Just acct -> do
-    taken <-
-      maybeFirstRow fromOnly $
-        DB.query
-          db
-          "SELECT 1 FROM users WHERE wallet_seed_id = ? AND wallet_account_index = ? AND user_id != ?"
-          (sId, fromIntegral acct :: Int64, userId)
-    case (taken :: Maybe Int64) of
-      Just _ -> pure $ Left "another profile uses this account"
-      Nothing -> do
-        bindUser db userId sId (fromIntegral acct)
-        -- the counter moves past it, so the next profile is not handed the same one
-        setNextAccountIndex db sId (fromIntegral acct + 1)
-        pure $ Right ()
+  Just acct
+    | acct >= 0x80000000 -> pure $ Left "account index too large"
+    | otherwise -> do
+        taken <-
+          maybeFirstRow fromOnly $
+            DB.query
+              db
+              "SELECT 1 FROM users WHERE wallet_seed_id = ? AND wallet_account_index = ? AND user_id != ?"
+              (sId, fromIntegral acct :: Int64, userId)
+        case (taken :: Maybe Int64) of
+          Just _ -> pure $ Left "another profile uses this account"
+          Nothing -> do
+            bindUser db userId sId (fromIntegral acct)
+            -- the counter moves past it, so the next profile is not handed the same one
+            setNextAccountIndex db sId (fromIntegral acct + 1)
+            pure $ Right ()
 
 -- | So two profiles cannot be handed the same account.
 takeAccountIndex :: DB.Connection -> SeedId -> IO Int64