Wire ↑↓↵ keyboard navigation in the command overlay

The footer chips already advertised ↑↓ to navigate and ↵ to open;
they now match reality.

- chrome::command_match exposes COMMAND_ACTIONS / matching_actions
  alongside the tab/history/bookmark match helpers and a
  CommandSelection enum + visible_command_rows that returns the flat
  ordered list of activatable rows. command_overlay drops its private
  copies of the action const and matcher and consumes them from the
  shared module so render and key handling share one source of truth.

- ElyShell tracks command_selected_index with command_select_next /
  command_select_prev (cyclic, no notify when index doesn't change)
  and activate_selected_command which dispatches the right shell call
  for the currently selected CommandSelection variant and dismisses
  the overlay. Dismissing command mode resets the index to 0.

- The shell root captures key_down via on_command_overlay_key_down.
  When the live snapshot's command_query starts with '>', up / down
  / enter run the matching helper and the event stops propagating so
  the omnibar input doesn't move its caret.

- Each rendered row receives a `selected` flag. The selected row gets
  the design's tinted bg + accent-bar on the left edge so the active
  result is unambiguous at any keyboard step.

crates/ely_app/src/shell/mod.rs

@@ -67,6 +67,7 @@ pub struct ElyShell {
     pub(crate) translucency_slider: Entity<SliderState>,
     pub(crate) workspace_picker_open: bool,
     pub(crate) sidebar_hover_expanded: bool,
+    pub(crate) command_selected_index: usize,
     download_action_error: Option<String>,
     download_clear_confirmation: bool,
     download_security_confirmation: Option<PendingDownloadFileAction>,
@@ -179,6 +180,7 @@ impl ElyShell {
             translucency_slider,
             workspace_picker_open: false,
             sidebar_hover_expanded: false,
+            command_selected_index: 0,
             download_action_error: None,
             download_clear_confirmation: false,
             download_security_confirmation: None,