feat: Add complete keyboard navigation system

Implemented a full keyboard navigation system with directional commands (C-h/j/k/l) for moving between views,
vim-style navigation (j/k for rows, h/l for columns), and TAB cycling.
Added table header navigation mode where users can move between columns and change sort order with keyboard shortcuts.
The navigation follows visual position: left sidebar, table, and filter bar can all be reached with directional keys.

Created a keymap system with context-aware bindings that resolve commands based on
current focus (Table, TableHeaders, FilterBar, Sidebar).
Commands are dispatched through a centralized handler that manages focus transitions and view state.
Added helper methods to cycle through column headers with proper wrapping.

Auto-select first task on load when tasks exist, and set initial window focus so keyboard shortcuts
work immediately without requiring a mouse click.

src/view/status_bar.rs

@@ -129,11 +129,7 @@ impl Render for StatusBar {
                             .items_center()
                             .gap_2()
                             .child(Label::new("✕").text_color(theme.error).text_sm())
-                            .child(
-                                Label::new(error.clone())
-                                    .text_color(theme.error)
-                                    .text_sm(),
-                            ),
+                            .child(Label::new(error.clone()).text_color(theme.error).text_sm()),
                     )
                     .child(
                         div()