Scaffold arrangement demo slice with CompareBar wiring

Add a self-contained demo build target for the Providers → Packages →
Comparison flow, deployable as a static SPA at /arrangement/.

- vite.demo.config.ts: per-slice build via --mode, base path flips for
  dev vs prod, output to dist-demo/<slice>/
- src/demo/: shared fixtures (7 providers across verified/tier3/tier2
  with real venue photography from brandassets) + Zustand basket store
  with ?compare= URL persistence
- Verified-provider packages now share the nine canonical Essentials
  line items per FA convention; only Optionals/Extras vary
- App-level CompareBar surfaces "Already added" / "Maximum 3" feedback
  via transient store error
- ProviderCard logo objectFit cover→contain so wide logos don't crop
- npm scripts demo:dev / demo:build, deps zustand + react-router-dom

.gitignore

@@ -1,5 +1,6 @@
 node_modules/
 dist/
+dist-demo/
 storybook-static/
 tokens/export/
 *.local
@@ -42,3 +43,6 @@ temp-db/
 
 # Root-level screenshots
 /*.png
+
+# IDE-specific
+*.code-workspace