refactor(app): the device's speech recognition moves from Learn into halkyon_app, with dictation into a text

Learn's `SpeechInput` and `DeviceSpeechInput` (ADR 0021, D67) are
halkyon_app's now, and its `FakeSpeechInput` is in `testing.dart`, so
Meditate's dream journal (ADR 0038, D15) listens through the same code.
Learn imports them from there and behaves as before: its composer and
its tests are unchanged but for the imports, and `DeviceSpeechInput()`
asks for what it asked before.

`DeviceSpeechInput` takes three options, off by default:
- `preferOnDevice`: on Android, recognition on the device where it can
  (Android 12 on), else offline; should that fail before a word is
  heard (not a pause), it listens as Android otherwise does from then
  on.
- `pauseFor`: how long a pause may last before the recogniser stops.
- `listenFor`: how long one listen may last at most.

`Dictation` writes what is heard where the cursor stands
(`DictationInsertion`: in place of a selection, set apart by a space),
shows the words so far, and listens again after each pause while it is
on, stopping by itself after three listens running heard nothing.

speech_to_text moves from Learn's pubspec to halkyon_app's, the lock
unchanged. Todo, Train and Meditate link the plugin now; their
generated plugin registrants (macOS, Windows) say so. An app declares
the microphone itself, and Todo and Train declare none, so nothing
there can listen.

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/test/feature/assistant/assistant_field_test.dart

@@ -1,5 +1,6 @@
 import 'package:flutter/foundation.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_app/testing.dart' show FakeSpeechInput;
 import 'package:halkyon_learn_flutter/src/core/content/rich_content.dart';
 import 'package:halkyon_learn_flutter/src/core/device_settings/model/device_settings.dart';
 import 'package:halkyon_learn_flutter/src/core/router/route_paths.dart';
@@ -17,7 +18,6 @@ import 'package:lucide_icons_flutter/lucide_icons.dart';
 import 'package:material_ui/material_ui.dart';
 
 import '../../support/fake_card_gateway.dart';
-import '../../support/fake_speech_input.dart';
 import '../../support/fake_studyset_gateway.dart';
 import '../../support/memory_device_settings_store.dart';
 import '../../support/pump_app.dart';