Refactor to View enum and add keybindings

Replace the `default_window` string field with a `View` enum throughout
the
application. Introduce a new keybinding module providing `Action`,
`KeyBinding` structs and helper functions, and integrate a keybindings
widget
into the main menu. Update the CLI to accept a `View` value enum and
adjust
imports and event handling accordingly. Add a release profile in
`Cargo.toml` with LTO, single codegen unit, higher optimisation level
and
binary stripping for smaller, faster builds.

src/app/widgets/keybindings.rs

@@ -0,0 +1,31 @@
+use ratatui::{
+    crossterm::event::KeyCode,
+    layout::Alignment,
+    style::Stylize,
+    text::{Line, Span},
+    widgets::{Block, Borders, Paragraph},
+};
+
+use crate::app::{View, binding_for_view};
+
+pub fn keybindings_widget(view: View) -> Paragraph<'static> {
+    let lines: Vec<Line> = binding_for_view(view)
+        .iter()
+        .map(|b| {
+            let key_span = match b.code {
+                KeyCode::Char(c) => Span::raw(c.to_string()),
+                other => Span::raw(format!("{:?}", other)),
+            }
+            .bold()
+            .red();
+
+            Line::from_iter([key_span, "\t - ".into(), b.description.into()])
+        })
+        .collect();
+
+    Paragraph::new(lines).alignment(Alignment::Left).block(
+        Block::default()
+            .borders(Borders::ALL)
+            .title("[ Keybindings ]"),
+    )
+}