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/ViewModels/UnlockViewModel.swift

@@ -66,8 +66,6 @@ final class UnlockViewModel: ObservableObject {
                 try session.unlock(url: url, password: password)
                 try keychainStore.save(password: password, for: keychainAccount)
                 password = ""
-            } catch KDBXError.invalidPassword {
-                errorMessage = "Incorrect password."
             } catch {
                 errorMessage = error.localizedDescription
             }