feat(learn): voice mode goes, as a spoken conversation isn't wanted (ADR 0021, D68 withdrawn)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

pubspec.yaml

@@ -59,16 +59,14 @@ dependency_overrides:
   # (`Logger.standard`, `Logger.verbose`, `Ansi`, `Progress`), the same in 0.5.2.
   # Drop this once flutter_launcher_icons allows 0.5.
   cli_util: ^0.5.2
-  # Three Android plugins patched for Kotlin built into the Android Gradle
+  # Two Android plugins patched for Kotlin built into the Android Gradle
   # plugin (android.builtInKotlin=true, which every app sets since 4 Nov
   # 2026): their latest releases still apply the Kotlin Gradle plugin
   # themselves, which Flutter warns will fail builds. Only android/build.gradle
   # differs from the release (third_party/README.md); drop each override once
   # its plugin's own release has migrated. flutter_web_auth_2 is
   # halkyon_oidc_flutter's, keyboard_height_plugin appflowy_editor's (Learn's
-  # notes), flutter_tts Learn's voice mode's.
-  flutter_tts:
-    path: third_party/flutter_tts
+  # notes).
   flutter_web_auth_2:
     path: third_party/flutter_web_auth_2
   keyboard_height_plugin: