fix: add manualChunks to resolve TDZ error in production build

[epuerta9]

Summary

Fixes #82 - UI JavaScript Error: ReferenceError on /agents/default page

Problem Analysis

The production UI bundle was throwing a Temporal Dead Zone (TDZ) error:

Uncaught ReferenceError: Cannot access 'Se' before initialization

This error occurred because:

1. The entire UI was bundled into a single large chunk (~3.2 MB)
2. Circular dependencies between modules caused variable initialization order issues
3. When JavaScript tried to access a variable before it was initialized, the TDZ violation occurred

Root Cause

In JavaScript, variables declared with let and const exist in a "Temporal Dead Zone" from the start of their scope until they are initialized. When Rollup/Vite bundles code with circular dependencies into a single chunk, the module evaluation order can cause some variables to be accessed before their initialization, triggering TDZ errors.

Solution

Added manualChunks configuration to ui/vite.config.ts to split the bundle into separate chunks:

Chunk	Libraries	Purpose
react-vendor	react, react-dom, react-router-dom	React core - loaded first
ui-vendor	@radix-ui/*, lucide-react, recharts	UI components - loaded after React
utils	date-fns, clsx, tailwind-merge, zod	Utility libraries

This ensures:

• ✅ Proper module initialization order
• ✅ Each chunk is evaluated independently
• ✅ Circular dependencies within app code don't affect vendor libraries
• ✅ Better caching - vendor chunks change less frequently

Files Modified

• ui/vite.config.ts - Added manualChunks configuration

Testing Notes

1. Build the UI: make build-ui or cd ui && npm run build
2. Verify multiple chunk files are created in internal/ui/static/assets/
3. Start the server: STN_DEV_MODE=true stn serve --local
4. Navigate to http://localhost:8585/agents/default
5. Confirm no JavaScript errors in browser console
6. Verify the agents page renders correctly

Additional Benefits

• Smaller main bundle size
• Better browser caching (vendor chunks rarely change)
• Improved initial load performance
• Easier debugging of production issues