feat(static): add clientInit option for pre-hydration initialization

[uhyo]

Summary

• Add a new optional clientInit configuration option that allows users to specify a module that runs on the client side before React hydration
• This enables client-side instrumentation like Sentry, analytics, or feature flags
• When not configured, the virtual module returns an empty string (no-op, zero bundle overhead)

Changes

• Add clientInit?: string to FunstackStaticOptions interface
• Add virtual:funstack/client-init virtual module handling in plugin
• Import client-init at top of client entry before React imports
• Add type declaration for the virtual module
• Document the new option in API docs
• Add e2e tests to verify execution timing

Usage

// vite.config.ts
funstackStatic({
  root: "./src/root.tsx",
  app: "./src/App.tsx",
  clientInit: "./src/client-init.ts", // optional
})

// src/client-init.ts - Side effects only, no exports needed
import * as Sentry from "@sentry/browser";

Sentry.init({
  dsn: "https://your-sentry-dsn",
  environment: import.meta.env.MODE,
});

Test plan

• Build succeeds (pnpm build)
• E2e tests pass (pnpm test:e2e)
• Verified clientInit runs before React hydration via timestamp comparison

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