fix(theme): wrap useTheme return with reactive() so toggle text updates immediately

useTheme returned a plain object, so template access to theme.isDark didn't track reactive dependencies. wrapping with reactive() ensures ref auto-unwrapping and proper change detection.

src/composables/useTheme.ts

@@ -1,4 +1,4 @@
-import { ref } from 'vue'
+import { ref, reactive } from 'vue'
 
 const STORAGE_KEY = 'mach-theme'
 const isDark = ref(false)
@@ -29,8 +29,8 @@ export function initTheme(): void {
 
 export function useTheme() {
   initTheme()
-  return {
+  return reactive({
     isDark,
     toggle: () => applyTheme(!isDark.value)
-  }
+  })
 }