Phase 0: scaffold Next.js, Tailwind tokens, Supabase clients, env validation

Next.js 16 App Router with strict TypeScript, Tailwind 4 theme tokens from
docs/UI.md, Vitest, and the three Supabase clients. Env vars are validated
with zod in next.config.ts so dev and build fail fast with a readable error.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

eslint.config.mjs

@@ -0,0 +1,18 @@
+import { defineConfig, globalIgnores } from "eslint/config";
+import nextVitals from "eslint-config-next/core-web-vitals";
+import nextTs from "eslint-config-next/typescript";
+
+const eslintConfig = defineConfig([
+  ...nextVitals,
+  ...nextTs,
+  // Override default ignores of eslint-config-next.
+  globalIgnores([
+    // Default ignores of eslint-config-next:
+    ".next/**",
+    "out/**",
+    "build/**",
+    "next-env.d.ts",
+  ]),
+]);
+
+export default eslintConfig;