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/index.html

@@ -152,6 +152,21 @@
 
                 <!-- Shared password fields -->
                 <div class="mb-2" id="add-wallet-password-section">
+                    <!-- Shown only when the profile already holds a wallet:
+                         each wallet has its own password (its own
+                         encryptedSecret), so a second wallet does not reuse
+                         the first one's. addWallet.js toggles this on screen
+                         entry from state.wallets.length, so it is constant
+                         while the screen is up and moves nothing. -->
+                    <p
+                        class="text-xs mb-2 border border-border border-dashed p-2 hidden"
+                        id="add-wallet-separate-password-note"
+                    >
+                        You already have a wallet. Each wallet has its own
+                        password: the one you choose here is only for this new
+                        wallet, and it need not match any password you already
+                        use.
+                    </p>
                     <label class="block mb-1">Choose a password</label>
                     <!-- The hint is swapped in place when the import tab
                          changes, and it sits directly above the password