refactor: move composables, stores, plugins under app/ srcDir; fix server path resolution

app/assets/css/main.css

@@ -0,0 +1,2 @@
+@import "tailwindcss";
+@import "@nuxt/ui";

nuxt.config.ts

@@ -13,7 +13,7 @@ export default defineNuxtConfig({
     strict: true,
   },
 
-  // Global CSS (Tailwind for @nuxt/ui)
+  // Global CSS
   css: ["~/assets/css/main.css"],
 
   // PostCSS configured separately

server/api/fixtures.get.ts

@@ -1,5 +1,5 @@
 import { defineEventHandler } from 'h3'
-import { useFixtureIO } from '~/composables/useFixtureIO'
+import { useFixtureIO } from '../../composables/useFixtureIO'
 
 export default defineEventHandler(() => {
   // Export snapshot of in-memory state

server/api/fixtures.post.ts

@@ -1,5 +1,5 @@
 import { defineEventHandler, readBody } from 'h3'
-import { useFixtureIO, type AppSnapshot } from '~/composables/useFixtureIO'
+import { useFixtureIO, type AppSnapshot } from '../../composables/useFixtureIO'
 
 export default defineEventHandler(async (event) => {
   const body = (await readBody(event)) as AppSnapshot