feat(devex): Align on Vite for production builds

Context

We switched the frontend dev server to Vite in July 2025 (PR #35247), but production builds still go through a custom esbuild pipeline (frontend/build.mjs + @posthog/esbuilder). That means two separate build systems with different configs, different behavior, and different bugs.

Goal

Use Vite as the single build tool for both development and production.

Scope / ideas

• Audit what the esbuild pipeline does beyond bundling — multi-entry-point builds (index, exporter, render-query, toolbar), metadata generation, parallel build orchestration
• Set up a Vite production build and validate output against current esbuild output (bundle size, chunking, etc.)
• Replace esbuild prod config with Vite
• Verify CI pipelines and deploy still work with the new build

Current state

• Vite 7.x for dev (frontend/vite.config.ts), esbuild 0.25.x for prod (frontend/build.mjs, common/esbuilder/utils.mjs)
• There was a previous attempt at Vite prod builds with an env flag POSTHOG_USE_VITE_PROD=1 (PRs #40432, #40431, Oct 2025) — both were closed without merging, marked WIP

Open questions

• Why were the previous Vite prod PRs abandoned? Blockers worth understanding before another attempt.

References

• Previous Vite prod attempt: https://github.com/PostHog/posthog/pull/40432
• Original Vite dev server: https://github.com/PostHog/posthog/pull/35247

Start by reading frontend/vite.config.ts and frontend/build.mjs, then inspect common/esbuilder/utils.mjs to inventory the current production pipeline. Review closed PR #40432 to understand prior blockers. Done means Vite handles the listed production entry points, output is compared with esbuild, and CI and deployment are verified.