Adds support for delegating S/MIME crypto operations to a separate
companion app over an AIDL service, paralleling the existing OpenPGP /
OpenKeychain integration. The reference provider is CipherMail
(com.ciphermail.android); other providers can implement the same API.

API surface (new module `plugins/smime-api/smime-api/`)
  - ISmimeService AIDL — execute(Intent, ParcelFileDescriptor, int) +
    createOutputPipe(int) for streaming bulk MIME data.
  - SmimeApi helper (sync + async execution wrappers, pipe management).
  - SmimeServiceConnection (bind lifecycle helper).
  - Parcelables: SmimeError, SmimeSignatureResult, SmimeDecryptionResult,
    SmimeCertificateInfo. All carry PARCELABLE_VERSION = 1.
  - Actions: CHECK_PERMISSION, DECRYPT_VERIFY, SIGN_AND_ENCRYPT,
    GET_CERTIFICATES, IMPORT_CERTIFICATE.

Receive-side integration
  - SmimeCryptoHelper (parallels MessageCryptoHelper for OpenPGP):
    detects S/MIME parts, binds to the provider, dispatches DECRYPT_VERIFY,
    surfaces RESULT_CODE_USER_INTERACTION_REQUIRED via PendingIntent so
    the host can launch the provider's passphrase dialog.
  - MessageCryptoStructureDetector.isSmimePart and helpers — detect
    application/pkcs7-mime and PKCS#7 multipart/signed.
  - CryptoResultAnnotation: new S/MIME fields and createSmime* factories.
  - MessageCryptoDisplayStatus: S/MIME signature/encryption mappings to
    the existing display-status badges.
  - MessageLoaderHelper, MessageCryptoPresenter, MessageViewInfoExtractor
    wired through.

Send-side integration
  - SmimeMessageBuilder (parallels PgpMessageBuilder): binds to the
    provider on a background thread, calls SIGN_AND_ENCRYPT, returns the
    wrapped MIME message for SMTP transport. Drafts bypass crypto.
  - MessageCompose: S/MIME branch in createMessageBuilder(), checked
    before PGP.
  - RecipientPresenter.asyncUpdateSmimeCertStatus: calls
    GET_CERTIFICATES on recipient changes; drives the compose lock-icon
    state (green = all certs present, red = missing).

Per-account configuration
  - LegacyAccount / LegacyAccountDto: smimeProvider field +
    isSmimeProviderConfigured.
  - LegacyAccountStorageHandler + DefaultLegacyAccountDataMapper:
    persist smimeProvider.
  - AccountSettingsFragment: S/MIME PreferenceScreen + provider picker.
  - SmimeAppSelectDialog: enumerates installed providers via
    SmimeApi.SERVICE_INTENT and lets the user choose. Binding always
    uses setPackage(account.smimeProvider) to avoid intent-filter
    interception.

Manifest plumbing
  - app-k9mail and app-thunderbird AndroidManifest: <queries> for
    ISmimeService discovery on Android 11+.
  - legacy/ui/legacy AndroidManifest: register SmimeAppSelectDialog.

Cross-process passphrase handshake
  - When the provider's keystore is locked it returns
    RESULT_CODE_USER_INTERACTION_REQUIRED with an immutable PendingIntent
    for its passphrase activity. Thunderbird launches via
    startIntentSenderForResult and retries on RESULT_OK. No inline
    prompting, no IPC timeouts.

Send/receive UX and per-identity signing
  - User-controllable Sign / Encrypt toggles with per-account defaults;
    S/MIME-enabled is persisted separately and a message is never sent
    silently unencrypted.
  - Encrypt-implies-sign enforced (no encrypt without a signature).
  - Per-identity signing: EXTRA_FROM carries the composing account's
    address on SIGN_AND_ENCRYPT so the provider signs with the matching
    certificate.
  - Separate signed / encrypted status icons and an "open in provider"
    action to view any encrypted or signed message in CipherMail.
  - Provider-unavailable handling: never lose a message; gate composer
    close on save completion; offer Drafts-folder assignment.
  - Send failures surface in a dismissible dialog rather than a Toast.
  - Set the intent extras class loader before reading Parcelable extras.
  - Test: RecipientPresenter.onSmimeCertCheckResult result branches.