Code splitting to reduce bundle size

[spydmobile]

The frontend builds into a single 2.1 MB JS chunk (dist/assets/index-*.js), well above Vite's 500 KB warning threshold.

Primary contributors:

• MapBox GL JS (~800KB-1MB minified)
• React + full application code in one bundle

Recommended approach:

• Lazy-load routes/features with dynamic import()
• Use build.rollupOptions.output.manualChunks to split vendor libs (MapBox, React) from app code
• Consider lazy-loading MapBox GL only when the map view is active

Priority: Low — optimization task, not a bug. Appropriate when approaching production deployment.