feat(settings): add home content customization and config injection

- Add home_content setting for custom homepage (HTML or iframe URL)
- Inject public settings into index.html to eliminate page flash
- Support ETag caching with automatic invalidation on settings update
- Add Vite plugin for dev mode settings injection
- Refactor HomeView to use appStore instead of local API calls

frontend/src/types/global.d.ts

@@ -0,0 +1,9 @@
+import type { PublicSettings } from '@/types'
+
+declare global {
+  interface Window {
+    __APP_CONFIG__?: PublicSettings
+  }
+}
+
+export {}

frontend/src/types/index.ts

@@ -73,6 +73,7 @@ export interface PublicSettings {
   api_base_url: string
   contact_info: string
   doc_url: string
+  home_content: string
   linuxdo_oauth_enabled: boolean
   version: string
 }