fix: say a second wallet's password is separate when one is chosen (closes #374)

The add-wallet screen offered only "Choose a password" while each wallet
keeps its own encrypted secret, so a second wallet silently accepted a
password different from the first with nothing marking it as separate. A
note now appears on that screen when the profile already holds a wallet,
saying each wallet has its own password and this one need not match any
already in use. It is shown only then — the first wallet has no other
password to differ from — and is decided on screen entry, so it does not
move the password fields. It promises no recovery or reset, staying
consistent with the no-password-reset design.

Model: opus-4-8

src/popup/views/addWallet.js

@@ -100,9 +100,24 @@ function clear() {
     $("add-wallet-phrase-warning").style.visibility = "hidden";
 }
 
+// Each wallet has its own password (its own encryptedSecret), so adding a
+// second wallet does not reuse the first one's. The note that says so is
+// only meaningful once a wallet exists — on the first wallet there is no
+// other password to be separate from — so it is shown only then. This is
+// decided on entry and stays put while the screen is up, so it does not
+// move the password fields the way a per-tab hint would.
+function updateSeparatePasswordNote() {
+    const hasExistingWallet = state.wallets.length > 0;
+    $("add-wallet-separate-password-note").classList.toggle(
+        "hidden",
+        !hasExistingWallet,
+    );
+}
+
 function show() {
     clear();
     switchMode("mnemonic");
+    updateSeparatePasswordNote();
     showView("add-wallet");
 }