fix: standardize error display to use showError/hideError helpers

Replace inconsistent error display patterns across views:

- addWallet.js: replace showFlash() with showError('add-wallet-error', ...)
- importKey.js: replace showFlash() with showError('import-key-error', ...)
- addressDetail.js: replace direct DOM manipulation of export-privkey-flash
  with showError/hideError('export-privkey-error', ...)
- deleteWallet.js: replace direct DOM manipulation of delete-wallet-flash
  with showError/hideError('delete-wallet-error', ...)
- index.html: add dedicated error divs with min-h-[1.25rem] for add-wallet,
  import-key views; rename flash divs to error divs for export-privkey and
  delete-wallet views with consistent styling

All password/validation errors now use the showError()/hideError() helper
pattern with min-h-[1.25rem] error divs to prevent layout shift.
Status/success messages (scanning, wallet deleted) remain as showFlash().

Closes #87

src/popup/views/addressDetail.js

@@ -2,6 +2,8 @@ const {
     $,
     showView,
     showFlash,
+    showError,
+    hideError,
     balanceLinesForAddress,
     addressDotHtml,
     addressTitle,
@@ -310,8 +312,7 @@ function init(_ctx) {
         $("export-privkey-address").textContent = addr.address;
         $("export-privkey-address").dataset.full = addr.address;
         $("export-privkey-password").value = "";
-        $("export-privkey-flash").classList.add("hidden");
-        $("export-privkey-flash").textContent = "";
+        hideError("export-privkey-error");
         $("export-privkey-password-section").classList.remove("hidden");
         $("export-privkey-result").classList.add("hidden");
         $("export-privkey-value").textContent = "";
@@ -321,8 +322,7 @@ function init(_ctx) {
     $("btn-export-privkey-confirm").addEventListener("click", async () => {
         const password = $("export-privkey-password").value;
         if (!password) {
-            $("export-privkey-flash").textContent = "Password is required.";
-            $("export-privkey-flash").classList.remove("hidden");
+            showError("export-privkey-error", "Password is required.");
             return;
         }
         const btn = $("btn-export-privkey-confirm");
@@ -343,10 +343,9 @@ function init(_ctx) {
             $("export-privkey-password-section").classList.add("hidden");
             $("export-privkey-value").textContent = privateKey;
             $("export-privkey-result").classList.remove("hidden");
-            $("export-privkey-flash").classList.add("hidden");
+            hideError("export-privkey-error");
         } catch {
-            $("export-privkey-flash").textContent = "Wrong password.";
-            $("export-privkey-flash").classList.remove("hidden");
+            showError("export-privkey-error", "Wrong password.");
         } finally {
             btn.disabled = false;
             btn.classList.remove("text-muted");