feat: add 200 character limit with counter to chat input field

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

android/app/src/main/java/com/tiangong/aiagent/ui/chat/ChatScreen.kt

@@ -256,13 +256,23 @@ fun ChatScreen(
                             }
                             OutlinedTextField(
                                 value = inputText,
-                                onValueChange = { inputText = it },
+                                onValueChange = { if (it.length <= 200) inputText = it },
                                 placeholder = {
                                     Text(if (uiState.editingMessageId != null) "编辑消息..." else "输入消息...")
                                 },
                                 modifier = Modifier.weight(1f),
                                 maxLines = 8,
-                                enabled = !uiState.isLoading && !uiState.isStreaming
+                                enabled = !uiState.isLoading && !uiState.isStreaming,
+                                supportingText = {
+                                    Text(
+                                        "${inputText.length}/200",
+                                        style = MaterialTheme.typography.bodySmall,
+                                        color = if (inputText.length >= 200)
+                                            MaterialTheme.colorScheme.error
+                                        else
+                                            MaterialTheme.colorScheme.onSurfaceVariant
+                                    )
+                                }
                             )
                             Spacer(modifier = Modifier.width(8.dp))