refactor: complete nesting simplification (category 3 - 100%)

Simplified deep nesting across the codebase using modern Rust patterns:
let-else guards, early returns, iterator chains, and extracted functions.

**Files improved:**

1. src/tdlib/messages.rs (44 → 28 spaces max indent)
   - fetch_missing_reply_info(): 7 → 2-3 levels
     * Extracted fetch_and_update_reply()
     * Used filter_map and iterator chains
   - get_chat_history() retry loop: 6 → 3 levels
     * Early continue for empty results
     * Used .flatten() instead of nested if-let

2. src/input/main_input.rs (40 → 36 spaces max indent)
   - handle_forward_mode(): 7 → 2-3 levels
     * Extracted forward_selected_message()
   - Reaction picker: 5 → 2-3 levels
     * Extracted send_reaction()
   - Scroll + load older: 6 → 2-3 levels
     * Extracted load_older_messages_if_needed()

3. src/config.rs (36 → 32 spaces max indent)
   - load_credentials(): 7 → 2-3 levels
     * Extracted load_credentials_from_file()
     * Extracted load_credentials_from_env()
     * Used ? operator for Option chains

**Results:**
- Max nesting in entire project: ≤32 spaces (8 levels)
- 8 new functions extracted for better separation of concerns
- All 343 tests passing ✅

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/tdlib/messages.rs

@@ -155,30 +155,32 @@ impl MessageManager {
             )
             .await;
 
-            match result {
-                Ok(tdlib_rs::enums::Messages::Messages(messages_obj)) => {
-                    if !messages_obj.messages.is_empty() {
-                        all_messages.clear(); // Очищаем предыдущие результаты
-                        for msg_opt in messages_obj.messages.iter().rev() {
-                            if let Some(msg) = msg_opt {
-                                if let Some(info) = self.convert_message(msg).await {
-                                    all_messages.push(info);
-                                }
-                            }
-                        }
-
-                        // Если получили непустой результат, прекращаем попытки
-                        // (TDLib вернёт столько сообщений, сколько доступно, до limit)
-                        break;
-                    }
-
-                    // Если сообщений мало, ждём перед следующей попыткой
-                    if attempt < max_attempts {
-                        sleep(Duration::from_millis(200)).await;
-                    }
-                }
+            let messages_obj = match result {
+                Ok(tdlib_rs::enums::Messages::Messages(obj)) => obj,
                 Err(e) => return Err(format!("Ошибка загрузки истории: {:?}", e)),
+            };
+
+            // Skip empty results
+            if messages_obj.messages.is_empty() {
+                // Ждём перед следующей попыткой
+                if attempt < max_attempts {
+                    sleep(Duration::from_millis(200)).await;
+                }
+                continue;
             }
+
+            // Convert messages using iterator chains (flatten removes None values)
+            all_messages.clear(); // Очищаем предыдущие результаты
+
+            for msg in messages_obj.messages.iter().rev().flatten() {
+                if let Some(info) = self.convert_message(msg).await {
+                    all_messages.push(info);
+                }
+            }
+
+            // Если получили непустой результат, прекращаем попытки
+            // (TDLib вернёт столько сообщений, сколько доступно, до limit)
+            break;
         }
 
         Ok(all_messages)
@@ -716,41 +718,60 @@ impl MessageManager {
     ///
     /// Вызывайте после загрузки истории чата для заполнения информации о цитируемых сообщениях.
     pub async fn fetch_missing_reply_info(&mut self) {
-        // Collect message IDs that need to be fetched
-        let mut to_fetch = Vec::new();
-        for msg in &self.current_chat_messages {
-            if let Some(ref reply) = msg.interactions.reply_to {
-                if reply.sender_name == "Unknown" {
-                    to_fetch.push(reply.message_id);
-                }
-            }
-        }
+        // Early return if no chat selected
+        let Some(chat_id) = self.current_chat_id else {
+            return;
+        };
 
-        // Fetch missing messages
-        if let Some(chat_id) = self.current_chat_id {
-            for message_id in to_fetch {
-                if let Ok(original_msg_enum) =
-                    functions::get_message(chat_id.as_i64(), message_id.as_i64(), self.client_id).await
-                {
-                    let tdlib_rs::enums::Message::Message(original_msg) = original_msg_enum;
-                    if let Some(orig_info) = self.convert_message(&original_msg).await {
-                            // Update the reply info
-                            for msg in &mut self.current_chat_messages {
-                                if let Some(ref mut reply) = msg.interactions.reply_to {
-                                    if reply.message_id == message_id {
-                                        reply.sender_name = orig_info.metadata.sender_name.clone();
-                                        reply.text = orig_info
-                                            .content
-                                            .text
-                                            .chars()
-                                            .take(50)
-                                            .collect::<String>();
-                                    }
-                                }
-                            }
-                    }
-                }
-            }
+        // Collect message IDs with missing reply info using filter_map
+        let to_fetch: Vec<MessageId> = self
+            .current_chat_messages
+            .iter()
+            .filter_map(|msg| {
+                msg.interactions
+                    .reply_to
+                    .as_ref()
+                    .filter(|reply| reply.sender_name == "Unknown")
+                    .map(|reply| reply.message_id)
+            })
+            .collect();
+
+        // Fetch and update each missing message
+        for message_id in to_fetch {
+            self.fetch_and_update_reply(chat_id, message_id).await;
         }
     }
+
+    /// Загружает одно сообщение и обновляет reply информацию.
+    async fn fetch_and_update_reply(&mut self, chat_id: ChatId, message_id: MessageId) {
+        // Try to fetch the original message
+        let Ok(original_msg_enum) =
+            functions::get_message(chat_id.as_i64(), message_id.as_i64(), self.client_id).await
+        else {
+            return;
+        };
+
+        let tdlib_rs::enums::Message::Message(original_msg) = original_msg_enum;
+        let Some(orig_info) = self.convert_message(&original_msg).await else {
+            return;
+        };
+
+        // Extract text preview (first 50 chars)
+        let text_preview: String = orig_info
+            .content
+            .text
+            .chars()
+            .take(50)
+            .collect();
+
+        // Update reply info in all messages that reference this message
+        self.current_chat_messages
+            .iter_mut()
+            .filter_map(|msg| msg.interactions.reply_to.as_mut())
+            .filter(|reply| reply.message_id == message_id)
+            .for_each(|reply| {
+                reply.sender_name = orig_info.metadata.sender_name.clone();
+                reply.text = text_preview.clone();
+            });
+    }
 }