refactor(tui): extract input handling helpers in App

[daisuke8000]

Summary

• Refactor monolithic handle_key() method (254 lines) into focused helper methods
• Improve code organization, testability, and maintainability

Changes

Before: Single monolithic method

fn handle_key(&mut self, code: KeyCode, modifiers: KeyModifiers) {
    // 254 lines of nested conditionals and match statements
}

After: Orchestrator with specialized handlers

fn handle_key(&mut self, code: KeyCode, modifiers: KeyModifiers) {
    if self.handle_overlay_keys(code) { return; }
    if self.handle_search_mode_keys(code, modifiers) { return; }
    if self.handle_global_keys(code, modifiers) { return; }
    if code == KeyCode::Enter && self.handle_enter_key() { return; }
    self.content.handle_key(...);
}

New helper methods

Method	Lines	Responsibility
handle_overlay_keys()	~35	RecordDetail, Help overlay handling
handle_search_mode_keys()	~30	Search input mode processing
handle_global_keys()	~50	Quit, help, tab switching, refresh
handle_escape_key()	~30	Navigation back logic
handle_refresh()	~15	Data reload for current view
handle_enter_key()	~65	Content-specific Enter actions
handle_key()	~25	Simple orchestrator

Benefits

• Testability: Each handler can be tested independently
• Debugging: Easier to locate issues in specific handlers
• Maintenance: Changes are isolated to relevant handlers
• Readability: Clear separation of concerns

Test plan

• cargo test --package mbr-tui - All 32 tests pass
• cargo clippy --package mbr-tui - No warnings
• cargo fmt - Applied