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/keymap/active_context.rs

@@ -0,0 +1,32 @@
+use super::ContextId;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum FocusTarget {
+    App,
+    Table,
+    TableHeaders,
+    SidebarProjects,
+    SidebarTags,
+}
+
+impl FocusTarget {
+    pub fn to_context(&self) -> ContextId {
+        match self {
+            Self::App => ContextId::Global,
+            Self::Table => ContextId::Table,
+            Self::TableHeaders => ContextId::TableHeaders,
+            Self::SidebarProjects => ContextId::SidebarProjects,
+            Self::SidebarTags => ContextId::SidebarTags,
+        }
+    }
+
+    pub fn is_sidebar(&self) -> bool {
+        matches!(self, Self::SidebarProjects | Self::SidebarTags)
+    }
+}
+
+impl Default for FocusTarget {
+    fn default() -> Self {
+        Self::App
+    }
+}