feat(learn): voice mode, a conversation spoken as Claude's "Let's talk" (ADR 0021, D68)

Beside the field's microphone, a button opens voice mode over the whole
screen: a glow rising from below, a big microphone, and +, who answers,
the speaker and End in one row. It listens with the device's recogniser,
asks what was heard in one kept conversation, reads the answer aloud
with the device's speech (flutter_tts, its Markdown left out) and
listens again. On Linux there is none, as there is no microphone; where
nothing reads aloud, answers are shown as text.

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/android/app/src/main/AndroidManifest.xml

@@ -96,5 +96,10 @@
         <intent>
             <action android:name="android.speech.RecognitionService"/>
         </intent>
+        <!-- The device's text-to-speech engines, which read an assistant's
+             answers aloud in voice mode (ADR 0021, D68). -->
+        <intent>
+            <action android:name="android.intent.action.TTS_SERVICE"/>
+        </intent>
     </queries>
 </manifest>