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/widgets/board.rs

@@ -42,7 +42,7 @@ impl Widget for BoardWidget<'_> {
                     .cells
                     .get(map_row * self.state.map_width + map_col)
                 {
-                    let mut cell: CellWidget = *template;
+                    let mut cell: CellWidget = template.clone();
 
                     if map_row == self.state.focused_cell.get_row()
                         && map_col == self.state.focused_cell.get_col()