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

MyPassCore/Sources/MyPassCore/KDBX/KDBXDocument.swift

@@ -18,7 +18,15 @@ public struct KDBXDocument {
             let tree = try KPKTree(contentsOf: url, key: key)
             return KDBXMapper.database(from: tree)
         } catch let err as NSError {
-            if err.domain == KPKErrorDomain && err.code == KPKErrorCode.passwordAndOrKeyfileWrong.rawValue {
+            let wrongKeyCodes: Set<Int> = [
+                Int(KPKErrorCode.passwordAndOrKeyfileWrong.rawValue),
+                // KDBX4's header HMAC check (which is password-derived) uses this same
+                // error code for both a wrong password/keyfile and true file corruption --
+                // KeePassKit doesn't distinguish the two. Wrong password is by far the
+                // more common cause, so we surface it as such.
+                Int(KPKErrorCode.kdbxHeaderHashVerificationFailed.rawValue),
+            ]
+            if err.domain == KPKErrorDomain && wrongKeyCodes.contains(err.code) {
                 throw KDBXError.invalidPassword
             }
             throw KDBXError.parseError(err.localizedDescription)

MyPassCore/Sources/MyPassCore/KDBX/KDBXError.swift

@@ -11,7 +11,7 @@ extension KDBXError: LocalizedError {
     public var errorDescription: String? {
         switch self {
         case .invalidPassword:
-            return "Incorrect password."
+            return "Incorrect password, or this vault file is corrupted."
         case .fileNotFound:
             return "The vault file could not be found."
         case .parseError(let message):