feat(learn): voice mode goes, as a spoken conversation isn't wanted (ADR 0021, D68 withdrawn)

The developer's review of 4 Nov 2026: "an actual conversation with the AI
chatbot is not desirable and necessary". The field's voice-mode button,
the screen it opened, reading answers aloud and its texts go, and with
them flutter_tts, its patched copy in third_party/ (one override fewer),
the TTS_SERVICE query in the manifest and the plugin's registrants. The
microphone stays and dictates into the question. ImageApproval is the
conversation sheet's own again, as only voice mode needed it.

Whats-New: Voice mode is gone; the microphone dictates into your question.
Whats-New-De: Der Sprachmodus ist weg; das Mikrofon diktiert in deine Frage.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/halkyon_learn_flutter/pubspec.yaml

@@ -61,9 +61,6 @@ dependencies:
   # Keeps the token of a Jellyfin server in the system's keyring; Serverpod's
   # sign-in keeps its session there with it already.
   flutter_secure_storage: ^10.0.0
-  # Reads an assistant's answers aloud in voice mode with the device's own
-  # text-to-speech (Android's, Apple's, Windows'; ADR 0021, D68).
-  flutter_tts: ^4.2.5
   fractional_indexing_dart: ^1.0.7
   # Pinned exactly: every device on one app version must compute the same
   # schedules.