feat(fonts): 前端字体工具函数、主题字体状态与设置类型

apps/app-frontend/src/helpers/system-fonts.ts

@@ -0,0 +1,19 @@
+import { invoke } from '@tauri-apps/api/core'
+
+import type { SystemFontFamily } from './font-family.ts'
+
+let cached: Promise<SystemFontFamily[]> | null = null
+
+/**
+ * Installed families are enumerated by the host once per session; installing a
+ * font while the launcher runs shows up after a restart.
+ */
+export function getSystemFontFamilies(): Promise<SystemFontFamily[]> {
+	cached ??= invoke<SystemFontFamily[]>('plugin:fonts|fonts_get_system_fonts').catch((error) => {
+		// A failed scan must not poison the session for a later retry.
+		cached = null
+		throw error
+	})
+
+	return cached
+}