build(react): enable React Compiler (React 19 auto-memoization) (F2)

Added babel-plugin-react-compiler to the Vite React plugin. The compiler
auto-memoizes components/hooks at build time, so manual useMemo/useCallback
become largely unnecessary going forward (existing ones are left in place —
harmless, and the compiler adds the rest). Safe here because the codebase is
rules-of-hooks clean (enforced by eslint-plugin-react-hooks).

Validated: production build succeeds and the e2e probe passes 17/17 with the
compiler on — every authed page renders axe-clean and Tracker/Summary/Analytics
reconciliation holds. (Build time ~2.2s -> ~6.2s from the compiler pass; the
runtime win is automatic memoization.) The compiler ESLint rule needs
eslint-plugin-react-hooks v6 — a future upgrade.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

package-lock.json

@@ -56,6 +56,7 @@
         "@testing-library/react": "^16.3.2",
         "@vitejs/plugin-react": "^4.3.3",
         "autoprefixer": "^10.4.20",
+        "babel-plugin-react-compiler": "^1.0.0",
         "concurrently": "^9.1.0",
         "eslint": "^9.39.4",
         "eslint-plugin-react-hooks": "^5.2.0",
@@ -6488,6 +6489,16 @@
         "@babel/core": "^7.4.0 || ^8.0.0-0 <8.0.0"
       }
     },
+    "node_modules/babel-plugin-react-compiler": {
+      "version": "1.0.0",
+      "resolved": "https://registry.npmjs.org/babel-plugin-react-compiler/-/babel-plugin-react-compiler-1.0.0.tgz",
+      "integrity": "sha512-Ixm8tFfoKKIPYdCCKYTsqv+Fd4IJ0DQqMyEimo+pxUOMUR9cVPlwTrFt9Avu+3cb6Zp3mAzl+t1MrG2fxxKsxw==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@babel/types": "^7.26.0"
+      }
+    },
     "node_modules/balanced-match": {
       "version": "4.0.4",
       "resolved": "https://registry.npmjs.org/balanced-match/-/balanced-match-4.0.4.tgz",

package.json

@@ -70,6 +70,7 @@
     "@testing-library/react": "^16.3.2",
     "@vitejs/plugin-react": "^4.3.3",
     "autoprefixer": "^10.4.20",
+    "babel-plugin-react-compiler": "^1.0.0",
     "concurrently": "^9.1.0",
     "eslint": "^9.39.4",
     "eslint-plugin-react-hooks": "^5.2.0",

vite.config.mjs

@@ -12,7 +12,14 @@ const apiPort   = process.env.API_PORT || process.env.PORT || 3000;
 
 export default defineConfig({
   plugins: [
-    react(),
+    // React Compiler (React 19): auto-memoizes components/hooks at build time, so
+    // manual useMemo/useCallback become largely unnecessary. Safe here because the
+    // codebase is rules-of-hooks clean (enforced by eslint-plugin-react-hooks).
+    react({
+      babel: {
+        plugins: [['babel-plugin-react-compiler', {}]],
+      },
+    }),
     VitePWA({
       registerType: 'autoUpdate',
       includeAssets: ['img/logo.png', 'img/pwa-192.png', 'img/pwa-512.png'],