fix: show every password error in its own line below the field (closes #493)

The add wallet screen reported a missing, short or mismatched password in
the flash line at the top of the popup, and the private key export,
recovery phrase and delete wallet screens each wrote to a line of their own
above the password field. All four now use showError() and hideError() with
a fixed-height error line below the field, as the send confirmation and
approval screens do. The line clears when the screen is shown again and
when the password is tried again. Other add wallet messages stay in the
flash line.

Model: opus-5-5

tests/exportPrivkey.test.js

@@ -207,7 +207,7 @@ describe("a decrypt still running when the screen is left", () => {
     });
 
     // Same hole on the failure path: a wrong-password error written after
-    // the wipe would restore the flash line on a screen the user has left.
+    // the wipe would restore the error line on a screen the user has left.
     test("never writes the failure message either", async () => {
         const { helpers, vault, exportPrivkey } = load();
         exportPrivkey.show(0, 0);
@@ -217,8 +217,10 @@ describe("a decrypt still running when the screen is left", () => {
         reveal.reject(new Error("decryption failed"));
         await reveal.pending;
 
-        expect(node("export-privkey-flash").textContent).toBe("");
-        expect(node("export-privkey-flash").style.visibility).toBe("hidden");
+        expect(node("export-privkey-password-error").textContent).toBe("");
+        expect(node("export-privkey-password-error").style.visibility).toBe(
+            "hidden",
+        );
     });
 });
 
@@ -260,7 +262,7 @@ describe("a reveal that is not interrupted", () => {
         await reveal.pending;
 
         expect(node("export-privkey-value").textContent).toBe("");
-        expect(node("export-privkey-flash").textContent).toBe(
+        expect(node("export-privkey-password-error").textContent).toBe(
             "That password is incorrect. Please try again.",
         );
     });