fix: standardize error display to use showError/hideError helpers

Replaces four inconsistent error display patterns with the centralized
showError()/hideError() helpers from helpers.js:

- approval.js: replace direct classList toggling on approve-tx-error and
  approve-sign-error with showError()/hideError()
- addressDetail.js: rename export-privkey-flash to export-privkey-error,
  use showError()/hideError() instead of direct DOM manipulation
- deleteWallet.js: rename delete-wallet-flash to delete-wallet-error,
  use showError()/hideError() instead of direct DOM with text-red-500
- addWallet.js: replace showFlash() validation errors with dedicated
  add-wallet-error div and showError() (keep showFlash for status msgs)
- importKey.js: replace showFlash() validation errors with dedicated
  import-key-error div and showError()
- index.html: add error divs with min-h-[1.25rem] for add-wallet,
  import-key views; update existing error divs to use consistent
  min-h-[1.25rem] class instead of hidden class

Closes #87

src/popup/index.html

@@ -104,7 +104,10 @@
                         class="border border-border p-1 w-full font-mono text-sm bg-bg text-fg"
                     />
                 </div>
-                <div id="add-wallet-error" class="text-xs mb-2 hidden"></div>
+                <div
+                    id="add-wallet-error"
+                    class="text-xs min-h-[1.25rem] mb-2"
+                ></div>
                 <button
                     id="btn-add-wallet-confirm"
                     class="border border-border px-2 py-1 hover:bg-fg hover:text-bg cursor-pointer"
@@ -163,7 +166,10 @@
                         class="border border-border p-1 w-full font-mono text-sm bg-bg text-fg"
                     />
                 </div>
-                <div id="import-key-error" class="text-xs mb-2 hidden"></div>
+                <div
+                    id="import-key-error"
+                    class="text-xs min-h-[1.25rem] mb-2"
+                ></div>
                 <button
                     id="btn-import-key-confirm"
                     class="border border-border px-2 py-1 hover:bg-fg hover:text-bg cursor-pointer"
@@ -368,7 +374,7 @@
                 </p>
                 <div
                     id="export-privkey-error"
-                    class="text-xs mb-2 hidden"
+                    class="text-xs min-h-[1.25rem] mb-2"
                 ></div>
                 <div id="export-privkey-password-section" class="mb-2">
                     <label class="block mb-1">Password</label>
@@ -940,7 +946,10 @@
                     <strong id="delete-wallet-name"></strong> is permanent. Any
                     funds will be unrecoverable without your recovery phrase.
                 </p>
-                <div id="delete-wallet-error" class="text-xs mb-2 hidden"></div>
+                <div
+                    id="delete-wallet-error"
+                    class="text-xs min-h-[1.25rem] mb-2"
+                ></div>
                 <div class="mb-2">
                     <label class="block mb-1">Password</label>
                     <input
@@ -1138,7 +1147,10 @@
                         class="border border-border p-1 w-full font-mono text-sm bg-bg text-fg"
                     />
                 </div>
-                <div id="approve-tx-error" class="text-xs hidden mb-2"></div>
+                <div
+                    id="approve-tx-error"
+                    class="text-xs min-h-[1.25rem] mb-2"
+                ></div>
                 <div class="flex justify-between">
                     <button
                         id="btn-approve-tx"
@@ -1201,7 +1213,10 @@
                         class="border border-border p-1 w-full font-mono text-sm bg-bg text-fg"
                     />
                 </div>
-                <div id="approve-sign-error" class="text-xs hidden mb-2"></div>
+                <div
+                    id="approve-sign-error"
+                    class="text-xs min-h-[1.25rem] mb-2"
+                ></div>
                 <div class="flex justify-between">
                     <button
                         id="btn-approve-sign"

src/popup/views/addWallet.js

@@ -99,7 +99,6 @@ function init(ctx) {
         state.hasWallet = true;
         await saveState();
         ctx.renderWalletList();
-        hideError("add-wallet-error");
         showView("main");
 
         // Scan for used HD addresses beyond index 0.