core: wallet export and delete act on the device key

The key is one per device, so both commands read it directly instead of
going through the active profile's account. A profile without an account
of its own can now export, and the delete error says that the key goes for
every profile on the device. The confirmation word is lowercased, as the
phrase is when imported.

Two known limitations are written down: account indexes restart at 0 after
an import, so profiles are bound in the order they ask for a key, and the
gap left by a hidden profile in the printed indexes shows that it exists.

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

src/Simplex/Chat/Store/Wallets.hs

@@ -6,7 +6,6 @@
 
 module Simplex.Chat.Store.Wallets
   ( getDeviceSeed,
-    getBoundAccount,
     getSeedAccounts,
     getOrCreateAccountRef,
     importSeed,