chore: gate legacy migration functions with #[cfg(windows)] (#31)

Replaced #[allow(dead_code)] with #[cfg(windows)] for
`migrate_from_numeric_id` and `get_theme_from_numeric_id` in
`src/icon_manager.rs`. These functions are only used for Windows-specific
legacy settings migration, and gating them correctly resolves dead code
warnings on other platforms.

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
Co-authored-by: bearice <270121+bearice@users.noreply.github.com>

Author: bearice

src/icon_manager.rs

@@ -224,7 +224,7 @@ impl IconManager {
     }
 
     // Migration support - convert old numeric IDs to string IDs
-    #[allow(dead_code)]
+    #[cfg(windows)]
     pub fn migrate_from_numeric_id(old_id: usize) -> String {
         let is_cat = (old_id & 2) == 0;
 
@@ -235,7 +235,7 @@ impl IconManager {
         }
     }
 
-    #[allow(dead_code)]
+    #[cfg(windows)]
     pub fn get_theme_from_numeric_id(old_id: usize) -> Theme {
         let is_dark = (old_id & 1) == 0;
         if is_dark {