refactor(CardTemplateEditor): add ViewModel + State infrastructure

Introduce CardTemplateEditorViewModel and CardTemplateEditorState as
the foundation for migrating CardTemplateEditor to use the Rust
backend's atomic updateNotetype() API.

Part 1 of migration, infrastructure only, no behavioral changes yet.

Author: xenonnn4w

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

@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2025 Snowiee <[email protected]>
+ *
+ * This program is free software; you can redistribute it and/or modify it under
+ * the terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 3 of the License, or (at your option) any later
+ * version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
+ * PARTICULAR PURPOSE. See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package com.ichi2.anki
+
+import com.ichi2.anki.libanki.CardOrdinal
+
+/** Encapsulates the entire state for [CardTemplateEditor] */
+data class CardTemplateEditorState(
+    /** Indicator if the UI should show a "loading" view to the user */
+    val isLoading: Boolean = true,
+    /** The currently selected template ordinal (0-based index) */
+    val currentTemplateOrd: CardOrdinal = 0,
+    /** The currently selected editor view (front/back/styling) */
+    val currentEditorViewId: Int = 0,
+    /** Error that occurred or null for no error */
+    val error: ReportableException? = null,
+    /** Simple transient messages in response to user actions or null for no message */
+    val message: UserMessage? = null,
+    /** Signal to close the activity after save or discard */
+    val shouldFinish: Boolean = false,
+) {
+    /** Simple message to be shown to the user */
+    enum class UserMessage {
+        CantDeleteLastTemplate,
+        CantAddTemplateToDynamic,
+        SaveSuccess,
+        DeletionWouldOrphanNote,
+    }
+
+    /**
+     * Wrapper around an exception produced in [CardTemplateEditor] with an extra flag about the
+     * exception being reportable or not.
+     */
+    data class ReportableException(
+        val source: Throwable,
+        /** true if this exception should be sent to [com.ichi2.anki.CrashReportService] */
+        val isReportable: Boolean = true,
+    )
+}

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

@@ -0,0 +1,57 @@
+/*
+ * Copyright (c) 2025 Snowiee <[email protected]>
+ *
+ * This program is free software; you can redistribute it and/or modify it under
+ * the terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 3 of the License, or (at your option) any later
+ * version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
+ * PARTICULAR PURPOSE. See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package com.ichi2.anki
+
+import androidx.lifecycle.ViewModel
+import com.ichi2.anki.libanki.CardOrdinal
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.asStateFlow
+import kotlinx.coroutines.flow.update
+
+class CardTemplateEditorViewModel : ViewModel() {
+    private val _state = MutableStateFlow(CardTemplateEditorState())
+    val state: StateFlow<CardTemplateEditorState> = _state.asStateFlow()
+
+    fun setCurrentTemplateOrd(ord: CardOrdinal) {
+        _state.update { it.copy(currentTemplateOrd = ord) }
+    }
+
+    fun setCurrentEditorView(viewId: Int) {
+        _state.update { it.copy(currentEditorViewId = viewId) }
+    }
+
+    fun setLoading(isLoading: Boolean) {
+        _state.update { it.copy(isLoading = isLoading) }
+    }
+
+    fun signalFinish() {
+        _state.update { it.copy(shouldFinish = true) }
+    }
+
+    fun clearMessage() {
+        _state.update { it.copy(message = null) }
+    }
+
+    fun clearError() {
+        _state.update { it.copy(error = null) }
+    }
+
+    fun clearShouldFinish() {
+        _state.update { it.copy(shouldFinish = false) }
+    }
+}

AnkiDroid/src/test/java/com/ichi2/anki/CardTemplateEditorViewModelTest.kt

@@ -0,0 +1,97 @@
+/*
+ * Copyright (c) 2025 Snowiee <[email protected]>
+ *
+ * This program is free software; you can redistribute it and/or modify it under
+ * the terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 3 of the License, or (at your option) any later
+ * version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
+ * PARTICULAR PURPOSE. See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package com.ichi2.anki
+
+import androidx.test.ext.junit.runners.AndroidJUnit4
+import com.ichi2.testutils.JvmTest
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Test
+import org.junit.runner.RunWith
+import kotlin.test.assertNull
+
+@RunWith(AndroidJUnit4::class)
+class CardTemplateEditorViewModelTest : JvmTest() {
+    private fun createViewModel() = CardTemplateEditorViewModel()
+
+    @Test
+    fun `initial state is loading with defaults`() {
+        val viewModel = createViewModel()
+        val state = viewModel.state.value
+
+        assertEquals(true, state.isLoading)
+        assertEquals(0, state.currentTemplateOrd)
+        assertEquals(0, state.currentEditorViewId)
+        assertNull(state.error)
+        assertNull(state.message)
+        assertFalse(state.shouldFinish)
+    }
+
+    @Test
+    fun `setCurrentTemplateOrd updates state correctly`() {
+        val viewModel = createViewModel()
+        viewModel.setCurrentTemplateOrd(2)
+        assertEquals(2, viewModel.state.value.currentTemplateOrd)
+    }
+
+    @Test
+    fun `setCurrentEditorView updates state correctly`() {
+        val viewModel = createViewModel()
+        viewModel.setCurrentEditorView(123)
+        assertEquals(123, viewModel.state.value.currentEditorViewId)
+    }
+
+    @Test
+    fun `setLoading updates isLoading state`() {
+        val viewModel = createViewModel()
+        viewModel.setLoading(false)
+        assertFalse(viewModel.state.value.isLoading)
+        viewModel.setLoading(true)
+        assertEquals(true, viewModel.state.value.isLoading)
+    }
+
+    @Test
+    fun `signalFinish sets shouldFinish flag`() {
+        val viewModel = createViewModel()
+        assertFalse(viewModel.state.value.shouldFinish)
+        viewModel.signalFinish()
+        assertEquals(true, viewModel.state.value.shouldFinish)
+    }
+
+    @Test
+    fun `clearShouldFinish resets the flag`() {
+        val viewModel = createViewModel()
+        viewModel.signalFinish()
+        assertEquals(true, viewModel.state.value.shouldFinish)
+        viewModel.clearShouldFinish()
+        assertFalse(viewModel.state.value.shouldFinish)
+    }
+
+    @Test
+    fun `clearMessage resets message to null`() {
+        val viewModel = createViewModel()
+        viewModel.clearMessage()
+        assertNull(viewModel.state.value.message)
+    }
+
+    @Test
+    fun `clearError resets error to null`() {
+        val viewModel = createViewModel()
+        viewModel.clearError()
+        assertNull(viewModel.state.value.error)
+    }
+}