feat: add mock writing assistant for UI testing

Temporary MockWritingAssistant that returns fake transformed text after
a short delay. Enabled via useMockAi flag in ShortNotePostViewModel.
Allows testing the AI writing help UI on devices without Gemini Nano.

TODO: Set useMockAi = false before shipping.

https://claude.ai/code/session_01RbCYGrbbapRMike8WQy41F

amethyst/src/main/java/com/vitorpamplona/amethyst/service/ai/MockWritingAssistant.kt

@@ -0,0 +1,97 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.amethyst.service.ai
+
+import kotlinx.coroutines.delay
+
+/**
+ * TODO: Remove before shipping. Debug-only mock for testing the AI writing help UI
+ * on devices without Gemini Nano / AICore support.
+ */
+class MockWritingAssistant : WritingAssistant {
+    override suspend fun checkAvailability(): WritingAssistantStatus = WritingAssistantStatus.Available
+
+    override suspend fun transform(
+        text: String,
+        tone: WritingTone,
+    ): WritingResult {
+        delay(800)
+
+        val transformed =
+            when (tone) {
+                WritingTone.CORRECT -> {
+                    correctMock(text)
+                }
+
+                WritingTone.REPHRASE -> {
+                    "Here's another way to put it: $text"
+                }
+
+                WritingTone.SHORTER -> {
+                    text
+                        .split(".")
+                        .firstOrNull()
+                        ?.trim()
+                        ?.plus(".") ?: text
+                }
+
+                WritingTone.ELABORATE -> {
+                    "$text Furthermore, this point deserves deeper consideration and nuance."
+                }
+
+                WritingTone.FRIENDLY -> {
+                    "Hey! $text Hope that makes sense! :)"
+                }
+
+                WritingTone.PROFESSIONAL -> {
+                    "I would like to bring to your attention the following: $text"
+                }
+
+                WritingTone.MORE_DIRECT -> {
+                    text.replace("I think ", "").replace("maybe ", "").replace("perhaps ", "")
+                }
+
+                WritingTone.PUNCHY -> {
+                    text.uppercase().replace(".", "!")
+                }
+
+                WritingTone.EMOJIFY -> {
+                    "$text \uD83D\uDE80\uD83D\uDD25\u2728"
+                }
+            }
+
+        return WritingResult(
+            originalText = text,
+            transformedText = transformed,
+            tone = tone,
+        )
+    }
+
+    private fun correctMock(text: String): String =
+        text
+            .replace("teh ", "the ")
+            .replace("dont ", "don't ")
+            .replace("cant ", "can't ")
+            .replace("wont ", "won't ")
+            .replace("i ", "I ")
+
+    override fun close() {}
+}

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/home/ShortNotePostViewModel.kt

@@ -44,6 +44,7 @@ import com.vitorpamplona.amethyst.model.Account
 import com.vitorpamplona.amethyst.model.LocalCache
 import com.vitorpamplona.amethyst.model.Note
 import com.vitorpamplona.amethyst.model.User
+import com.vitorpamplona.amethyst.service.ai.MockWritingAssistant
 import com.vitorpamplona.amethyst.service.ai.WritingAssistant
 import com.vitorpamplona.amethyst.service.ai.WritingAssistantFactory
 import com.vitorpamplona.amethyst.service.ai.WritingAssistantStatus
@@ -297,6 +298,9 @@ open class ShortNotePostViewModel :
     var wantsAnonymousPost by mutableStateOf(false)
 
     // AI Writing Help
+    // TODO: Remove useMockAi before shipping. Set to true to test UI without Gemini Nano.
+    private val useMockAi = true
+
     var wantsAiHelp by mutableStateOf(false)
     var aiResult by mutableStateOf<WritingResult?>(null)
     var aiStatus by mutableStateOf<WritingAssistantStatus>(WritingAssistantStatus.Unavailable)
@@ -305,7 +309,12 @@ open class ShortNotePostViewModel :
 
     fun initWritingAssistant(context: android.content.Context) {
         if (writingAssistant == null) {
-            writingAssistant = WritingAssistantFactory.create(context)
+            writingAssistant =
+                if (useMockAi) {
+                    MockWritingAssistant()
+                } else {
+                    WritingAssistantFactory.create(context)
+                }
             viewModelScope.launch(Dispatchers.IO) {
                 aiStatus = writingAssistant?.checkAvailability() ?: WritingAssistantStatus.Unavailable
             }