fix(backend): stop showing the clinician null, NaN and the wrong failure

Two ways a voice failure described itself wrongly.

describe() built the quoted-back text from fields that are all nullable on
the wire, and toVoiceIntent casts rather than checks — so a half-classified
deadline rendered as “null null” — not a usable date, and an offset with no
amount as “+NaN day”. Blank is already handled by the sheet; it now falls
back to that.

The DTO's constraints resolved to unrelated codes: maxLength fell through
to VALIDATION_FIELD_REQUIRED, so an oversized recording said a field was
missing, and isIn maps to VALIDATION_LANGUAGE_INVALID, so an unsupported
container said the language was invalid. Both now name their own code —
the validation factory already returns a message verbatim when it is itself
a known ErrorCode, so this needs no change to the shared mapping.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

frontend/messages/fa.json

@@ -1268,6 +1268,7 @@
     "VOICE_MIC_DENIED": "دسترسی به میکروفون مسدود شده است. در تنظیمات مرورگر اجازه دهید و دوباره تلاش کنید.",
     "VOICE_NOT_AVAILABLE": "ثبت گفتاری هنوز برای این زبان در دسترس نیست.",
     "VOICE_CLIP_TOO_LONG": "مدت ضبط بیش از حد است. لطفاً کمتر از دو دقیقه صحبت کنید.",
+    "VOICE_UNSUPPORTED_FORMAT": "قالب این ضبط پشتیبانی نمی‌شود.",
     "VOICE_ASR_FAILED": "تبدیل گفتار به متن انجام نشد. لطفاً دوباره تلاش کنید.",
     "VOICE_EXTRACT_FAILED": "اطلاعات درمان از روی گفتار استخراج نشد.",
     "VOICE_NOTHING_RECOGNIZED": "گفتاری شناسایی نشد. میکروفون را بررسی کنید و دوباره تلاش کنید.",