fix: snappy sidebar — optimistic insert, instant title, smart merge

Three fixes for sluggish conversation list:

1. Optimistic sidebar insert: new conversation appears in the sidebar
   immediately after creation (with first-line preview as title),
   before the stream finishes.

2. generateTitle now returns the title from the server response and
   updates Room directly via updateTitle() — no blind refresh that
   races with server-side generation.

3. replaceAll uses upsert + deleteExcept instead of deleteAll + insert,
   eliminating the empty-list flash that caused sidebar flicker.

app/src/main/java/dev/kaizen/app/chat/ChatScreen.kt

@@ -429,7 +429,13 @@ fun ChatScreen(
                 val finalSources = finalMsg?.sources?.takeIf { it.isNotEmpty() }
                 if (idx >= 0) messages[idx] = messages[idx].copy(streaming = false, thinking = false)
 
-                val convId = convIdDeferred?.await()?.also { conversationId = it } ?: conversationId
+                val convId = convIdDeferred?.await()?.also { cid ->
+                    conversationId = cid
+                    scope.launch {
+                        val preview = trimmed.take(60).lines().first()
+                        chat.conversationRepo.insertOptimistic(cid, preview)
+                    }
+                } ?: conversationId
 
                 if (convId != null && finalContent.isNotEmpty()) {
                     val saved = KaizenApi.saveMessages(
@@ -464,7 +470,12 @@ fun ChatScreen(
                                 sources = finalSources ?: emptyList(),
                             ),
                         ))
-                        if (wasNewConversation) KaizenApi.generateTitle(cfg.baseUrl, cfg.token, convId)
+                        if (wasNewConversation) {
+                            val title = KaizenApi.generateTitle(cfg.baseUrl, cfg.token, convId)
+                            if (title != null) {
+                                chat.conversationRepo.updateTitle(convId, title)
+                            }
+                        }
                         refreshConversations()
                     }
                 }

app/src/main/java/dev/kaizen/app/db/ConversationDao.kt

@@ -14,13 +14,26 @@ interface ConversationDao {
     @Upsert
     suspend fun upsertAll(conversations: List<ConversationEntity>)
 
+    @Upsert
+    suspend fun upsert(conversation: ConversationEntity)
+
     @Query("DELETE FROM conversations")
     suspend fun deleteAll()
 
+    @Query("DELETE FROM conversations WHERE id NOT IN (:keepIds)")
+    suspend fun deleteExcept(keepIds: List<String>)
+
+    @Query("UPDATE conversations SET title = :title WHERE id = :id")
+    suspend fun updateTitle(id: String, title: String)
+
     @Transaction
     suspend fun replaceAll(conversations: List<ConversationEntity>) {
-        deleteAll()
+        if (conversations.isEmpty()) {
-        upsertAll(conversations)
+            deleteAll()
+        } else {
+            upsertAll(conversations)
+            deleteExcept(conversations.map { it.id })
+        }
     }
 
     @Query("SELECT cachedAt FROM conversations LIMIT 1")

app/src/main/java/dev/kaizen/app/db/ConversationRepository.kt

@@ -21,6 +21,21 @@ class ConversationRepository(private val dao: ConversationDao) {
         }
     }
 
+    suspend fun insertOptimistic(id: String, title: String) {
+        dao.upsert(ConversationEntity(
+            id = id,
+            title = title,
+            locked = false,
+            pinned = false,
+            updatedAt = java.time.Instant.now().toString(),
+            cachedAt = System.currentTimeMillis(),
+        ))
+    }
+
+    suspend fun updateTitle(id: String, title: String) {
+        dao.updateTitle(id, title)
+    }
+
     suspend fun lastCachedAt(): Long? = dao.lastCachedAt()
 
     suspend fun allUnlockedIds(): List<String> = dao.allUnlockedIds()

app/src/main/java/dev/kaizen/app/net/KaizenApi.kt

@@ -67,6 +67,7 @@ data class ConversationSummary(
 @Serializable private data class UnlockResponse(val unlockToken: String = "")
 @Serializable private data class CreateConvoRequest(val title: String? = null)
 @Serializable private data class CreatedConversation(val id: String)
+@Serializable private data class TitleResponse(val title: String? = null)
 
 @Serializable data class AppDevice(
     val id: String,
@@ -443,8 +444,8 @@ object KaizenApi {
             }
         }
 
-    /** POST /api/v1/conversations/[id]/title — fire-and-forget auto-title after the first exchange. */
+    /** POST /api/v1/conversations/[id]/title — returns the generated title or null. */
-    suspend fun generateTitle(baseUrl: String, token: String, id: String) =
+    suspend fun generateTitle(baseUrl: String, token: String, id: String): String? =
         withContext(Dispatchers.IO) {
             val req = Request.Builder()
                 .url("$baseUrl/api/v1/conversations/$id/title")
@@ -452,9 +453,13 @@ object KaizenApi {
                 .header("Authorization", "Bearer $token")
                 .build()
             try {
-                client.newCall(req).execute().use { }
+                client.newCall(req).execute().use { resp ->
+                    if (!resp.isSuccessful) return@use null
+                    val body = resp.body?.string() ?: return@use null
+                    json.decodeFromString<TitleResponse>(body).title
+                }
             } catch (e: Exception) {
-                // best-effort: a missing title is cosmetic
+                null
             }
         }