refactor: prepare handlers structure for future input refactoring

Preparation for splitting large input file (#2):
- Created src/input/handlers/ structure (7 modules)
  - clipboard.rs (~100 lines) - clipboard operations extracted
  - global.rs (~90 lines) - global commands (Ctrl+R/S/P/F) extracted
  - Stubs: profile.rs, search.rs, modal.rs, messages.rs, chat_list.rs
- main_input.rs remains monolithic (1139 lines)
  - Attempted full migration broke navigation - rolled back
  - Handlers remain as preparation for gradual migration

Updated documentation:
- REFACTORING_OPPORTUNITIES.md: #2.1 status updated
- CONTEXT.md: Added lesson about careful refactoring

Lesson learned: Critical input logic requires careful step-by-step
refactoring with functionality verification after each step.

Tests: 563 passed, 0 failed

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

src/input/handlers/search.rs

@@ -0,0 +1,16 @@
+//! Search mode input handling (chat search and message search)
+
+use crate::app::App;
+use crossterm::event::KeyEvent;
+
+/// Обрабатывает ввод в режиме поиска чатов
+pub async fn handle_chat_search_input(app: &mut App, key: KeyEvent) {
+    // TODO: Implement chat search input handling
+    let _ = (app, key);
+}
+
+/// Обрабатывает ввод в режиме поиска сообщений
+pub async fn handle_message_search_input(app: &mut App, key: KeyEvent) {
+    // TODO: Implement message search input handling
+    let _ = (app, key);
+}