Route-level code splitting for faster mobile first paint (#106)

The whole app shipped in one 578 KB chunk, so a phone on cell data
downloaded and parsed everything — the canvas editor, gestures, geometry,
every page — before the login screen could paint.

- Lazy-load the heavy/deep routes via React.lazy: the editor (its
  GardenCanvas + use-gesture + geometry are the biggest surface), the
  public garden view, plants, settings, register. Login and the gardens
  list stay eager (entry points — no fallback flash on landing). AppShell
  wraps <Outlet> in a Suspense boundary.
- One `vendor` manualChunk for all node_modules so the rarely-changing
  libraries cache across app deploys while the tiny app chunk churns.
  Kept as a SINGLE chunk deliberately: splitting react-dom/scheduler into
  their own chunk reorders module init across chunk boundaries and breaks
  React 19 at load ("Cannot set 'Activity' of undefined") — verified that
  failure and backed it out.

Result: app entry chunk 578 KB → 39 KB; vendor 421 KB (cached); the editor
(43 KB) + canvas (17 KB) only download when you open a garden. No more
>500 KB chunk warning.

Verified live against the embedded binary: /gardens loads with only
index+vendor; opening a garden lazy-fetches the editor chunk and renders;
console clean; the embed serves the hashed split chunks + SPA fallback fine.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01H3zbym8Doka2d7D48maSgZ

web/vite.config.ts

@@ -29,6 +29,20 @@ export default defineConfig(({ mode }) => {
     build: {
       outDir: 'dist',
       sourcemap: true,
+      rollupOptions: {
+        output: {
+          // One vendor chunk for ALL node_modules: it's rarely-changing, so it
+          // caches across app deploys while the tiny app chunk churns. Kept as a
+          // SINGLE chunk on purpose — splitting react-dom/scheduler into their own
+          // chunk reorders their module init across chunk boundaries and breaks
+          // React 19 at load ("Cannot set 'Activity' of undefined"). The heavy
+          // routes are code-split separately via React.lazy (router.tsx), which is
+          // where the real first-paint win is.
+          manualChunks(id) {
+            return id.includes('node_modules') ? 'vendor' : undefined
+          },
+        },
+      },
     },
   }
 })