chore: migrate to @vigdorov/* shared configs (eslint, prettier, typescript, vite)

Replace project-local ESLint, Prettier, TypeScript, and Vite configs
with shared packages from dev-configs monorepo.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/vite.config.ts

@@ -1,16 +1,18 @@
-import { defineConfig } from 'vite'
-import react from '@vitejs/plugin-react'
+import {spa} from '@vigdorov/vite-config';
+import {mergeConfig} from 'vite';
 
-// https://vite.dev/config/
-export default defineConfig({
-  plugins: [react()],
-  server: {
-    port: 4000,
-    proxy: {
-      '/api': {
-        target: 'http://localhost:4001',
-        changeOrigin: true,
-      },
+export default mergeConfig(
+    spa({
+        port: 4000,
+    }),
+    {
+        server: {
+            proxy: {
+                '/api': {
+                    target: 'http://localhost:4001',
+                    changeOrigin: true,
+                },
+            },
+        },
     },
-  },
-})
+);