Migrate frontend to Next 16

[MaxGhenis]

Summary

• replace the Vite frontend entrypoint with a Next 16 App Router shell
• upgrade to React 19.2, Tailwind 4, and @policyengine/ui-kit 0.9
• keep Vercel's Python API functions and export the frontend statically from frontend/out

Substantive review

• PR Expand cliff inputs to match Atlanta Fed tools #7 materially expanded benefit inputs and URL state; I reviewed it and kept it because it improved the product surface without conflicting with the migration.
• PR Show state-specific program labels #8 was stacked on Expand cliff inputs to match Atlanta Fed tools #7; I cherry-picked only its state-program label fix into Show state-specific program labels #9 and merged that, avoiding stale stacked-branch baggage.
• This PR is then a pure framework transition over the reviewed main branch.

Verification

• cd frontend && bun run test
• cd frontend && bun run build
• env -u UV_FROZEN uv run --with pytest --with policyengine-us pytest

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Project	Deployment	Actions	Updated (UTC)
cliff-watch	Ready	Preview, Comment	May 10, 2026 8:05am