fix: show every password error in its own fixed-height line (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. On the add wallet screen the line sits beside the
Import button, which keeps its place at 360x600. 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

src/popup/index.html

@@ -207,12 +207,22 @@
                         class="border border-border p-1 w-full font-mono text-sm bg-bg text-fg"
                     />
                 </div>
-                <button
-                    id="btn-add-wallet-confirm"
-                    class="border border-border px-2 py-1 hover:bg-fg hover:text-bg cursor-pointer"
-                >
-                    Import
-                </button>
+                <!-- The error line sits beside Import, not above it: at
+                     360x600 the button already starts near the bottom of
+                     the popup, and a line of its own would push it below
+                     the fold. The longest error fits on one line here. -->
+                <div class="flex items-center gap-2">
+                    <button
+                        id="btn-add-wallet-confirm"
+                        class="border border-border px-2 py-1 hover:bg-fg hover:text-bg cursor-pointer"
+                    >
+                        Import
+                    </button>
+                    <div
+                        id="add-wallet-password-error"
+                        class="text-xs min-h-[1.25rem] invisible"
+                    ></div>
+                </div>
             </div>
 
             <!-- ============ MAIN VIEW: ALL WALLETS & ADDRESSES ============ -->
@@ -396,10 +406,6 @@
                     Warning: anyone with this private key can access and
                     transfer all funds from this address. Never share it.
                 </p>
-                <div
-                    id="export-privkey-flash"
-                    class="text-xs mb-2 min-h-[1.25rem] invisible"
-                ></div>
                 <div id="export-privkey-password-section" class="mb-2">
                     <label class="block mb-1">Password</label>
                     <input
@@ -408,9 +414,13 @@
                         class="border border-border p-1 w-full font-mono text-sm bg-bg text-fg"
                         placeholder="Enter your password to continue"
                     />
+                    <div
+                        id="export-privkey-password-error"
+                        class="text-xs mt-2 mb-2 min-h-[1.25rem] invisible"
+                    ></div>
                     <button
                         id="btn-export-privkey-confirm"
-                        class="border border-border px-2 py-1 hover:bg-fg hover:text-bg cursor-pointer mt-2"
+                        class="border border-border px-2 py-1 hover:bg-fg hover:text-bg cursor-pointer"
                     >
                         Reveal
                     </button>
@@ -1116,10 +1126,6 @@
                     <strong id="delete-wallet-name"></strong> is permanent. Any
                     funds will be unrecoverable without your recovery phrase.
                 </p>
-                <div
-                    id="delete-wallet-flash"
-                    class="text-xs text-red-500 mb-2 min-h-[1.25rem] invisible"
-                ></div>
                 <div class="mb-2">
                     <label class="block mb-1">Password</label>
                     <input
@@ -1129,6 +1135,10 @@
                         placeholder="Enter your password to confirm"
                     />
                 </div>
+                <div
+                    id="delete-wallet-password-error"
+                    class="text-xs mb-2 min-h-[1.25rem] invisible"
+                ></div>
                 <button
                     id="btn-delete-wallet-confirm"
                     class="border border-border text-red-500 px-2 py-1 hover:bg-fg hover:text-bg cursor-pointer"
@@ -1273,10 +1283,6 @@
                     this wallet, from any device, without your password. Never
                     type them into a website and never show them to anyone.
                 </div>
-                <div
-                    id="show-phrase-flash"
-                    class="text-xs text-red-500 mb-2 min-h-[1.25rem] invisible"
-                ></div>
                 <div id="show-phrase-password-section" class="mb-2">
                     <label class="block mb-1">Password</label>
                     <input
@@ -1285,9 +1291,13 @@
                         class="border border-border p-1 w-full font-mono text-sm bg-bg text-fg"
                         placeholder="Enter your password to continue"
                     />
+                    <div
+                        id="show-phrase-password-error"
+                        class="text-xs mt-2 mb-2 min-h-[1.25rem] invisible"
+                    ></div>
                     <button
                         id="btn-show-phrase-reveal"
-                        class="border border-border px-2 py-1 hover:bg-fg hover:text-bg cursor-pointer mt-2"
+                        class="border border-border px-2 py-1 hover:bg-fg hover:text-bg cursor-pointer"
                     >
                         Reveal
                     </button>