fix(storybook): type the font path plugin as a Vite Plugin

this.warn() in the hm-ui-theme-font-path transform failed tsc -b with
TS2339 because the untyped object literal made `this` the literal itself.
Annotate it as Vite's Plugin so `this` is the plugin context. Font URL
replacement and the warning behavior are unchanged.

.storybook/main.ts

@@ -2,6 +2,7 @@ import type { StorybookConfig } from "@storybook/react-vite"
 import { readFileSync, realpathSync } from "node:fs"
 import { fileURLToPath } from "node:url"
 import path from "node:path"
+import type { Plugin } from "vite"
 
 // Checkouts that share another checkout's node_modules (a linked git worktree whose node_modules is a junction) also
 // inherit that checkout's node_modules/@hm-design/* links. Those links point at the other checkout's packages/ui and its
@@ -35,7 +36,7 @@ const localUiAliases = [
 const UI_DIST_FONT_URL = /[^"'()\\\s]*\/ui\/dist\/fonts\/NotoSansKR-variable\.ttf/g
 const FONT_STATIC_URL = "/fonts/NotoSansKR-variable.ttf"
 
-const hmUiThemeFontPath = {
+const hmUiThemeFontPath: Plugin = {
   name: "hm-ui-theme-font-path",
   apply: "serve" as const,
   enforce: "post" as const,