feat: add password reveal toggle, friendlier KDBX4 wrong-password message

Adds an eye-icon toggle to the master password field (UnlockView) so
users can verify what they typed before submitting.

Also maps KDBX4's ERROR_HEADER_HASH_VERIFICATION_FAILED -- surfaced
via the header HMAC check, which is password-derived -- to
KDBXError.invalidPassword alongside the existing
passwordAndOrKeyfileWrong check. KeePassKit uses this same error code
for both a wrong password/keyfile and genuine file corruption (it
doesn't distinguish the two), so the message hedges: "Incorrect
password, or this vault file is corrupted." Found while testing
against a real KDBX4 vault, which was surfacing this as a raw,
unreadable NSError string before the previous commit's LocalizedError
fix, and as *no* friendly message at all before this one (the error
code wasn't in the recognized set yet).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WYqycDFsynHH9VnnK7LNSf

MyPass/Views/UnlockView.swift

@@ -59,9 +59,7 @@ struct UnlockView: View {
                     .foregroundStyle(.secondary)
             }
 
-            SecureField("Master Password", text: $vm.password)
-                .textFieldStyle(.roundedBorder)
-                .onSubmit(vm.unlockWithPassword)
+            PasswordField(text: $vm.password, onSubmit: vm.unlockWithPassword)
 
             Button("Unlock", action: vm.unlockWithPassword)
                 .buttonStyle(.bordered)
@@ -87,3 +85,42 @@ struct UnlockView: View {
         }
     }
 }
+
+/// A password field with a trailing eye button to reveal/hide the typed value.
+private struct PasswordField: View {
+    @Binding var text: String
+    var onSubmit: () -> Void
+
+    @State private var isRevealed = false
+
+    var body: some View {
+        HStack {
+            Group {
+                if isRevealed {
+                    TextField("Master Password", text: $text)
+                } else {
+                    SecureField("Master Password", text: $text)
+                }
+            }
+            .textFieldStyle(.plain)
+            .autocorrectionDisabled()
+            #if os(iOS)
+            .textInputAutocapitalization(.never)
+            #endif
+            .onSubmit(onSubmit)
+
+            Button {
+                isRevealed.toggle()
+            } label: {
+                Image(systemName: isRevealed ? "eye.slash" : "eye")
+                    .foregroundStyle(.secondary)
+            }
+            .buttonStyle(.plain)
+        }
+        .padding(8)
+        .overlay(
+            RoundedRectangle(cornerRadius: 6)
+                .stroke(Color.secondary.opacity(0.3), lineWidth: 1)
+        )
+    }
+}