refactor: modularize architecture

refactors the application structure to move away from a monolithic
`main.rs`.

- **Domain Models**: Extracted `State`, `Message`, `FileAction`, and
`ViewAction` into `state.rs` and `message.rs`. Enforced encapsulation on
`State` using private fields and public accessors.
- **Controller Layer**: Created `handler.rs` to handle business logic
and I/O operations, transforming the main `update` loop into a pure
event router.
- **Component System**: Split the UI into composable, stateless
functions in `components/` (tabs, editor, status_bar, action_bar),
removing the 100+ line view function.
- **File Logic**: Encapsulated file operations and safe path handling
within `file.rs`, removing fragile `expect` calls from the UI layer.
- **Utilities**: Centralized static configuration, keybindings, and I/O
helpers into `constants.rs`, `key_bindings.rs`, and `io.rs`.

This change decouples the view from the model and makes the core
application logic testable

src/constants.rs

@@ -0,0 +1,8 @@
+use iced::Font;
+
+pub const CUSTOM_FONT_BYTES: &[u8] = include_bytes!("./fonts/CaskaydiaCoveNFM-Regular.ttf");
+pub const CUSTOM_FONT: Font = Font::with_name("CaskaydiaCove Nerd Font Mono");
+pub const DEFAULT_EDITOR_FONT_SIZE: u32 = 16;
+pub const MAX_EDITOR_FONT_SIZE: u32 = 80;
+pub const MIN_EDITOR_FONT_SIZE: u32 = 12;
+pub const ICON_BYTES: &[u8] = include_bytes!("./images/icon.ico");