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/skills_config.rs

@@ -1,6 +1,6 @@
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct SkillsConfigState {
-  pub id: usize,
-  pub name: &'static str,
-  pub selected_skill: usize,
+    pub id: usize,
+    pub name: &'static str,
+    pub selected_skill: usize,
 }