fix: explain a stored non-master xprv wallet instead of throwing at signing time (closes #234)

An xprv wallet imported before non-master keys were refused holds a key
whose depth is greater than zero. Its addresses were derived by applying
the Ethereum path beneath that key, so they are not the addresses the key
produces under the standard path, and signing for them now throws — on the
send screen, with no explanation.

Detect it at wallet-list render time instead. An xprv wallet stores the
neutered node four levels below the imported key, so a master import stores
a depth-4 xpub and a depth-d import stores depth d + 4; the stored xpub is
therefore an exact read on the imported key's depth and needs no password.

The wallet list renders a named explanation under the wallet's name, the
"+" button is withheld, and send, dapp transaction approval, dapp message
signing and private-key export all refuse before asking for a password.
getSignerForAddress remains the backstop and now says why in a sentence.

The copy states what is true and nothing more: the addresses do descend
from the key that was imported, so it neither promises the funds are safe
nor implies anything was lost. The wallet is not deleted or rewritten.

README.md

@@ -436,7 +436,11 @@ The core hierarchy is **Wallets → Addresses**:
       multi-address behavior as an HD wallet, including the "+" button and the
       address scan on import, but imported from an extended private key rather
       than a recovery phrase. It therefore has no recovery phrase to display or
-      back up.
+      back up. Only a master key may be imported; an xprv wallet already in
+      storage that was imported from a non-master key is detected from the depth
+      of its stored `xpub` by `src/shared/walletDefects.js`, explained in the
+      wallet list, and blocked from signing, sending and private-key export. It
+      is never deleted or rewritten.
 - An **address** holds ETH and ERC-20 tokens.
 - The user can have multiple wallets, each with multiple addresses (HD) or a
   single address (key).