refactor: convert FieldFilter to an interface

Assisted-by: Claude Opus 4.7

Author: david-allison

AnkiDroid/src/main/java/com/ichi2/anki/TtsVoices.kt

@@ -24,6 +24,8 @@ package com.ichi2.anki
 
 import android.speech.tts.TextToSpeech
 import android.speech.tts.Voice
+import com.ichi2.anki.TtsVoices.availableLocaleData
+import com.ichi2.anki.TtsVoices.availableLocales
 import com.ichi2.anki.common.coroutines.applicationScope
 import com.ichi2.anki.i18n.normalize
 import com.ichi2.anki.i18n.toAnkiTwoLetterCode
@@ -219,7 +221,7 @@ object TtsVoices {
 /**
  * `{{tts-voices:}}` A filter which lists all available TTS Voices for the current engine
  */
-class TtsVoicesFieldFilter : TemplateManager.FieldFilter() {
+class TtsVoicesFieldFilter : TemplateManager.FieldFilter {
     // modified from libAnki: tts.py: on_tts_voices
     override fun apply(
         fieldText: String,

libanki/src/main/java/com/ichi2/anki/libanki/TemplateManager.kt

@@ -292,8 +292,8 @@ class TemplateManager {
      * Custom filters can check `filterName` to decide whether it should modify
      * `fieldText` or not before returning it
      */
-    abstract class FieldFilter {
-        abstract fun apply(
+    interface FieldFilter {
+        fun apply(
             fieldText: String,
             fieldName: String,
             filterName: String,