refactor(tui): Split app.rs into smaller modules

[lowhung]

Summary

app.rs contains the entire state machine. Split for maintainability.

Proposed Structure

• app/mod.rs - Main App struct and core state
• app/navigation.rs - View navigation logic
• app/state.rs - State management
• app/actions.rs - User action handlers
• app/filter.rs - Filter/search logic

Benefits

• Easier to navigate codebase
• Smaller, focused modules
• Better testability