chore(web/build): split vendor chunks — 705 kB main bundle → 246 kB

Single-bundle build was tripping vite's 500 kB warning per chunk and
forcing every user to re-download the entire app on every deploy.
Manual chunks split the bundle along natural library boundaries so:

- Rarely-changing vendor libs (react-dom, react-router, lucide-react,
  asciinema-player) cache across deploys.
- App code lives in its own `index-*.js` that's the only chunk that
  changes when we ship feature work.

Split shape (manualChunks fn in vite.config.ts):
- charts   — recharts + d3-*
- player   — asciinema-player
- icons    — lucide-react
- router   — react-router / react-router-dom
- react-dom, react
- vendor   — everything else in node_modules

Resulting bundle sizes (gzip):
  index (app):       246 kB  (gz 63)
  react-dom:         182 kB  (gz 57)
  player:            176 kB  (gz 65)
  router:             42 kB  (gz 15)
  vendor:             36 kB  (gz 14)
  icons:              29 kB  (gz 10)

Every chunk under the 600 kB ceiling we now set explicitly. The old
~705 kB single-chunk deploy is gone. No code changes — config only.

decnet_web/vite.config.ts

@@ -12,4 +12,30 @@ export default defineConfig({
       },
     },
   },
+  build: {
+    // Split heavy third-party libs into their own chunks so the main
+    // bundle stays small and the rarely-changing vendor code stays
+    // cacheable across deploys. Recharts + asciinema-player + lucide
+    // together made up most of the weight that was tripping the 500kB
+    // warning.
+    rollupOptions: {
+      output: {
+        manualChunks: (id: string) => {
+          if (!id.includes('node_modules')) return undefined
+          // d3-* ships alongside recharts as its plotting engine —
+          // grouping them keeps tree-shaken subsets together.
+          if (id.includes('recharts') || id.includes('/d3-')) return 'charts'
+          if (id.includes('asciinema-player')) return 'player'
+          if (id.includes('lucide-react')) return 'icons'
+          if (id.includes('react-router')) return 'router'
+          if (id.includes('react-dom')) return 'react-dom'
+          if (id.includes('/react/') || id.endsWith('/react')) return 'react'
+          return 'vendor'
+        },
+      },
+    },
+    // Legitimate ceiling for any single chunk after splitting; anything
+    // larger is a real bloat regression worth investigating.
+    chunkSizeWarningLimit: 600,
+  },
 })