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>

packages/halkyon_app/pubspec.yaml

@@ -69,6 +69,11 @@ dependencies:
   # libsodium's API, as halkyon_keys pins it (the same version, exactly):
   # opening it on each isolate the account's keys are worked on.
   sodium: 4.1.0+1
+  # What the person says, written (SpeechInput, Dictation; Halkyon Learn's
+  # ADR 0021, D67, Halkyon Meditate's ADR 0038, D15): the device's own speech
+  # recognition (Android's, Apple's, Windows'); none on Linux. An app that
+  # listens declares the microphone and its usage texts itself.
+  speech_to_text: ^7.5.0
   url_launcher: ^6.3.2
 
 dev_dependencies: