fix: deliver client-side templates to Pages Router clients

[vklimontovich]

What

Client-side templates (used to compile script insertions) were only available to App Router clients. App Router's NextlyticsServer collects them from config and passes them via the ctx prop; Pages Router's getNextlyticsProps runs inside getInitialProps with no access to config, so its clients never received them.

This includes templates in every /api/event response and merges them on the client from whichever source supplies them (ctx prop for App Router, response for Pages Router), so script insertions compile on both routers and survive client-side navigations.

It also fixes getNextlyticsProps throwing on client-side navigations, where _app's getInitialProps re-runs without a req. It now returns an empty context in that case.

Changes

• api-handler.ts — thread a collectTemplates callback through the handler; include templates in all /api/event responses.
• client.tsx — hold templates in state, merging from the ctx prop and the /api/event response.
• pages-router.ts — return an empty context when req is absent instead of throwing.
• middleware.ts / server.tsx — wire up the collectTemplates callback.
• types.ts — add optional templates to ClientRequestResult.

Testing

• bun run typecheck passes.

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