fix: standardize error display to use showError/hideError helpers

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

- approval.js: replace direct DOM classList toggling for approve-tx-error
  and approve-sign-error with showError/hideError calls
- addressDetail.js: replace export-privkey-flash direct DOM with
  showError/hideError using renamed export-privkey-error element
- deleteWallet.js: replace delete-wallet-flash direct DOM with
  showError/hideError using renamed delete-wallet-error element
- addWallet.js: replace showFlash() validation errors with dedicated
  add-wallet-error div and showError/hideError calls
- importKey.js: replace showFlash() validation errors with dedicated
  import-key-error div and showError/hideError calls
- index.html: add error divs for add-wallet and import-key views,
  rename export-privkey-flash to export-privkey-error,
  rename delete-wallet-flash to delete-wallet-error,
  remove inconsistent text-red-500 class

Closes #87

src/popup/index.html

@@ -104,10 +104,7 @@
                         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 min-h-[1.25rem] mb-2"
-                ></div>
+                <div id="add-wallet-error" class="text-xs mb-2 hidden"></div>
                 <button
                     id="btn-add-wallet-confirm"
                     class="border border-border px-2 py-1 hover:bg-fg hover:text-bg cursor-pointer"
@@ -166,10 +163,7 @@
                         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 min-h-[1.25rem] mb-2"
-                ></div>
+                <div id="import-key-error" class="text-xs mb-2 hidden"></div>
                 <button
                     id="btn-import-key-confirm"
                     class="border border-border px-2 py-1 hover:bg-fg hover:text-bg cursor-pointer"
@@ -374,7 +368,7 @@
                 </p>
                 <div
                     id="export-privkey-error"
-                    class="text-xs min-h-[1.25rem] mb-2"
+                    class="text-xs mb-2 hidden"
                 ></div>
                 <div id="export-privkey-password-section" class="mb-2">
                     <label class="block mb-1">Password</label>
@@ -946,10 +940,7 @@
                     <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 min-h-[1.25rem] mb-2"
-                ></div>
+                <div id="delete-wallet-error" class="text-xs mb-2 hidden"></div>
                 <div class="mb-2">
                     <label class="block mb-1">Password</label>
                     <input
@@ -1147,10 +1138,7 @@
                         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 min-h-[1.25rem] mb-2"
-                ></div>
+                <div id="approve-tx-error" class="text-xs hidden mb-2"></div>
                 <div class="flex justify-between">
                     <button
                         id="btn-approve-tx"
@@ -1213,10 +1201,7 @@
                         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 min-h-[1.25rem] mb-2"
-                ></div>
+                <div id="approve-sign-error" class="text-xs hidden mb-2"></div>
                 <div class="flex justify-between">
                     <button
                         id="btn-approve-sign"

src/popup/views/addWallet.js

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