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/en.json

@@ -1267,6 +1267,7 @@
     "VOICE_MIC_DENIED": "Microphone access was blocked. Allow it in your browser settings and try again.",
     "VOICE_NOT_AVAILABLE": "Voice entry is not available for this language yet.",
     "VOICE_CLIP_TOO_LONG": "That recording is too long. Please keep it under two minutes.",
+    "VOICE_UNSUPPORTED_FORMAT": "That recording format is not supported on this device.",
     "VOICE_ASR_FAILED": "Could not turn the recording into text. Please try again.",
     "VOICE_EXTRACT_FAILED": "Could not read the treatment details from the recording.",
     "VOICE_NOTHING_RECOGNIZED": "No speech was recognised. Check the microphone and try again.",