feat: background prefetch of all conversation messages for offline access

On app start, after syncing the conversation list, prefetch messages
for any conversation not yet cached in Room. This ensures all chats
are readable offline, not just previously opened ones.

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

@@ -166,6 +166,10 @@ fun ChatScreen(
         if (errors.isNotEmpty()) {
             loadError = errors.joinToString(" · ")
         }
+        scope.launch {
+            val ids = chat.conversationRepo.allUnlockedIds()
+            chat.messageRepo.prefetchMissing(cfg.baseUrl, cfg.token, ids)
+        }
     }
 
     // Warm the HTTP/2 connection pool on every app resume (covers background → foreground

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

@@ -25,4 +25,7 @@ interface ConversationDao {
 
     @Query("SELECT cachedAt FROM conversations LIMIT 1")
     suspend fun lastCachedAt(): Long?
+
+    @Query("SELECT id FROM conversations WHERE locked = 0")
+    suspend fun allUnlockedIds(): List<String>
 }

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

@@ -22,4 +22,6 @@ class ConversationRepository(private val dao: ConversationDao) {
     }
 
     suspend fun lastCachedAt(): Long? = dao.lastCachedAt()
+
+    suspend fun allUnlockedIds(): List<String> = dao.allUnlockedIds()
 }

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

@@ -18,4 +18,7 @@ interface MessageDao {
 
     @Query("DELETE FROM messages WHERE conversationId = :convId")
     suspend fun deleteByConversation(convId: String)
+
+    @Query("SELECT DISTINCT conversationId FROM messages")
+    suspend fun cachedConversationIds(): List<String>
 }

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

@@ -21,4 +21,14 @@ class MessageRepository(private val dao: MessageDao) {
     suspend fun cacheMessages(convId: String, messages: List<MessageEntity>) {
         dao.upsertAll(messages)
     }
+
+    suspend fun prefetchMissing(baseUrl: String, token: String, conversationIds: List<String>) {
+        val cached = dao.cachedConversationIds().toSet()
+        for (id in conversationIds) {
+            if (id in cached) continue
+            try {
+                fetchAndCache(baseUrl, token, id)
+            } catch (_: Exception) { }
+        }
+    }
 }