Update formatting to use 4‑space indentation

- Adjust rustfmt.toml to set tab_spaces = 4.
- Reformat imports, struct definitions, function bodies, and match arms
  across the project to match the new style (aligned fields, added
  spacing, etc.).
- Ensure all code complies with the updated rustfmt settings.

src/app/states/settings.rs

@@ -2,12 +2,12 @@ use crate::app::states::{PerkDecks, skirmish_states::GameMode};
 
 #[derive(Debug, Clone, PartialEq)]
 pub struct SettingsState {
-  pub username: String,
-  pub game_mode: GameMode,
-  pub perk_deck: PerkDecks,
-  pub starting_wood: u16,
-  pub starting_metal: u16,
-  pub supply_limit: u8,
-  pub xp_modifier: f32,
-  pub skill_points_limit: u16,
+    pub username: String,
+    pub game_mode: GameMode,
+    pub perk_deck: PerkDecks,
+    pub starting_wood: u16,
+    pub starting_metal: u16,
+    pub supply_limit: u8,
+    pub xp_modifier: f32,
+    pub skill_points_limit: u16,
 }