Add side panel and refactor title helpers

Introduce a `side_panel` flag throughout the skirmish state, board
creation,
and cell area calculation to enable a detachable side panel. Refactor
the
title helper to own its strings and use a static separator, updating the
single‑title helper accordingly. Add a new `SidePanelWidget` and expose
the
`skirmish_main_area_layout` helper. Extend keybindings with `Tab` and
`ShiftTab` actions under a new `Opener` group. Update structures and
units
to implement a `get_name` method and adjust related traits and imports.

src/app/states/skirmish_states/units/units_trait.rs

@@ -3,8 +3,5 @@ use crate::app::states::Players;
 pub trait Unit {
     fn get_owner(&self) -> Players;
     fn get_tag(&self) -> char;
-}
-
-pub trait OptionalUnit {
-    fn try_get_tag(&self) -> char;
+    fn get_name(&self) -> &'static str;
 }