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/message.rs

@@ -0,0 +1,71 @@
+use std::fmt::Display;
+
+use iced::widget::text_editor;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum FileAction {
+    New,
+    Save,
+    SaveAs,
+    Open,
+    Close(Option<usize>),
+}
+
+impl FileAction {
+    pub const ALL: &'static [FileAction] = &[
+        FileAction::New,
+        FileAction::Save,
+        FileAction::SaveAs,
+        FileAction::Open,
+        FileAction::Close(None),
+    ];
+}
+
+impl Display for FileAction {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            FileAction::New => write!(f, "New file"),
+            FileAction::Save => write!(f, "Save"),
+            FileAction::SaveAs => write!(f, "Save as... "),
+            FileAction::Open => write!(f, "Open"),
+            FileAction::Close(_) => write!(f, "Close"),
+        }
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum ViewAction {
+    Increase,
+    Decrease,
+    Reset,
+    TogglePreview,
+}
+
+impl ViewAction {
+    pub const ALL: &'static [ViewAction] = &[
+        ViewAction::Increase,
+        ViewAction::Decrease,
+        ViewAction::Reset,
+        ViewAction::TogglePreview,
+    ];
+}
+
+impl Display for ViewAction {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            ViewAction::Decrease => write!(f, "Decrease font"),
+            ViewAction::Increase => write!(f, "Increase font"),
+            ViewAction::Reset => write!(f, "Reset font"),
+            ViewAction::TogglePreview => write!(f, "Toggle preview"),
+        }
+    }
+}
+
+#[derive(Debug, Clone)]
+pub enum Message {
+    Edit(text_editor::Action),
+    FileActionSelected(FileAction),
+    ViewActionSelected(ViewAction),
+    SwitchTab(usize),
+    LinkClicked(String),
+}