Replace iframe with Vercel reverse proxy support

[PavelMakarchuk]

Summary

Closes #106

Prepare the tracker for a crawler-only reverse proxy on policyengine.org. Search engines and social scrapers will receive the tracker HTML directly (with proper canonical URLs, structured data, and sitemap on policyengine.org). Regular users continue to see the tracker inside the existing iframe with the full policyengine.org nav/footer — no visual change for humans.

This repo (tracker)

• Rename Vite assetsDir from assets/ to _tracker/ to avoid path collisions with app-v2's /assets/
• Add BASE_PATH module that detects proxied (/us/state-legislative-tracker) vs standalone routing
• Update all pushState calls in App.jsx and StatePanel.jsx to use BASE_PATH
• Update prerender canonical URLs, sitemap, robots.txt to point to policyengine.org
• Update og:image to policyengine.org domain
• Update Modal static mount from /assets to /_tracker
• Bump APP_VERSION to v22

Companion changes needed in policyengine-app-v2

• vercel.json: Add rewrite rules for /_tracker/:path* → Modal (assets)
• app/middleware.ts: For crawler user-agents hitting /us/state-legislative-tracker/*, let the Vercel rewrite serve Modal HTML directly instead of the app shell. Regular users still get the iframe page.
• No changes to iframe page or homepage — users see the same UI as before

Architecture

Crawlers:  policyengine.org/us/state-legislative-tracker/GA
           → middleware detects bot UA → passes through
           → Vercel rewrite → modal.run/GA → pre-rendered HTML with SEO tags

Users:     policyengine.org/us/state-legislative-tracker/GA
           → middleware returns app shell → iframe loads tracker as before

Assets:    policyengine.org/_tracker/index-*.js
           → Vercel rewrite → modal.run/_tracker/index-*.js

Backward compatibility

All existing URLs continue working:

• modal.run/GA — standalone, unchanged (BASE_PATH = '')
• policyengine.org/.../state-legislative-tracker — iframe for users, proxy for crawlers
• Old hash URLs (#GA) — redirect logic preserved
• All previously shared links remain valid

Deploy order

1. This repo first → merge, deploy to Modal, verify standalone works
2. app-v2 second → add Vercel rewrites + middleware crawler split

Test plan

• npm run build → dist/_tracker/ has JS/CSS, no dist/assets/
• Built index.html references /_tracker/index-*.js
• Dev server: click state → URL /GA, click bill → /GA/bill-id, back button works
• After Modal deploy: standalone modal.run/GA still works
• After app-v2 deploy: curl -A Googlebot .../state-legislative-tracker/GA → tracker HTML with canonical policyengine.org URL
• After app-v2 deploy: browser visit → iframe with policyengine.org nav (unchanged)

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