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/api/admin/settings.ts

@@ -22,6 +22,7 @@ export interface SystemSettings {
   api_base_url: string
   contact_info: string
   doc_url: string
+  home_content: string
   // SMTP settings
   smtp_host: string
   smtp_port: number
@@ -55,6 +56,7 @@ export interface UpdateSettingsRequest {
   api_base_url?: string
   contact_info?: string
   doc_url?: string
+  home_content?: string
   smtp_host?: string
   smtp_port?: number
   smtp_username?: string