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

README.md

@@ -1444,14 +1444,17 @@ view would leave a wallet one click from deletion.
       without it, the lost-password route on DeleteWallet is the first they
       would hear of it. The hint line reserves its height, so switching tabs
       cannot move the password fields under the pointer.
+    - Error line
     - "Import" button
 - **Transitions**:
     - "Import" with a valid entry and a matching password of at least 12
       characters → creates the wallet, clears the navigation stack, and →
       **Home**. The phrase and xprv modes then scan for further used addresses
       and report the count as a flash message.
-    - "Import" with an invalid entry, a duplicate wallet or address, or a short
-      or mismatched password → flash message, no screen change
+    - "Import" with a missing, short or mismatched password → full-sentence
+      error on the error line, no screen change
+    - "Import" with an invalid entry or a duplicate wallet or address → flash
+      message, no screen change
     - "Back" → previous screen (Welcome, Home, or Settings)
 
 #### AddressDetail (`address`)
@@ -1490,8 +1493,8 @@ view would leave a wallet one click from deletion.
       copy)
     - Warning that anyone holding the private key can transfer all funds from
       the address
-    - Error line
-    - Password input and "Reveal" button, shown until the key is revealed
+    - Password input, error line and "Reveal" button, shown until the key is
+      revealed
     - The private key on a highlighted background, tap to copy, shown only after
       the password has been accepted
 - **Transitions**:
@@ -1829,8 +1832,8 @@ view would leave a wallet one click from deletion.
     - Wallet name
     - Warning box stating that anyone holding these words can take everything in
       the wallet, from any device, without the password
-    - Error line
-    - Password input + "Reveal" button, shown until the password is accepted
+    - Password input, error line and "Reveal" button, shown until the password
+      is accepted
     - The recovery phrase itself, in full and click-to-copy, shown only after a
       correct password and in place of the password prompt
 - **Transitions**:
@@ -1859,8 +1862,8 @@ view would leave a wallet one click from deletion.
     - "Back" button, "Delete Wallet" heading
     - Warning naming the wallet and stating that deletion is permanent and any
       funds are unrecoverable without the recovery phrase
-    - Error line
     - Password input
+    - Error line
     - "Confirm Delete" button
     - An underlined "I have lost my password" control
 - **Transitions**: