Add zoom support and emoji keybinding icons

- CellWidget now stores a ZoomLevel, renders size‑dependent text, and
  provides
  methods to set the zoom level and build its block.
- BoardState creates cells with the current zoom level, adds a helper to
  get a
  mutable cell, and updates each cell’s zoom level in `zoom_change`.
- KeybindingsWidget constructor now takes the widget height, calculates
  vertical padding to centre groups, and adds left borders between
  groups.
- Views (default, main_menu, skirmish) pass the area height to the new
  constructor.
- Keybinding descriptions are replaced with emoji symbols (🔈, 🔊, 🔉).
- BoardWidget clones the cell template instead of dereferencing it.

src/app/views/main_menu.rs

@@ -93,6 +93,6 @@ pub fn main_menu_view(app: &App, area: Rect, buf: &mut Buffer) {
     }
 
     {
-        KeybindingsWidget::new(ACTIONS).render(keybindings_area, buf);
+        KeybindingsWidget::new(ACTIONS, keybindings_area.height).render(keybindings_area, buf);
     }
 }