fix: the defective xprv wallet notice says the user needs their own copy of the key (closes #255)

The notice told the user that these addresses are reachable by software
that repeats the same derivation, while the same wallet blocks Export
Private Key, so there was no way to take that route from here. The
second paragraph now names private key export among the disabled
controls and says the addresses can be reached only with the extended
private key the user imported, if they kept a copy of it. No behaviour
changes. A test checks the new wording.

Model: opus-5-5

tests/walletDefects.test.js

@@ -153,6 +153,18 @@ describe("the explanatory copy", () => {
         expect(all).not.toMatch(/\blost\b|\bstolen\b|\bgone\b/i);
     });
 
+    // The wallet holds the imported key but will not export it, so the copy
+    // must not send the user to other software without saying they need
+    // their own copy of that key.
+    test("it says only the user's own copy of the imported key reaches the addresses", () => {
+        const body = defect.paragraphs.join(" ");
+
+        expect(body).toMatch(/private key export are disabled/);
+        expect(body).toMatch(
+            /only with the extended private key you imported, if you kept a copy of it/,
+        );
+    });
+
     test("it says the wallet is not deleted and what the user can do", () => {
         const body = defect.paragraphs.join(" ");