feat: web SPA build target + UX overhaul + public deployment setup

- Replace Electron IPC with native fetch + AbortController (VITE_VALHALLA_URL)
- Add vite.web.config.ts for standalone SPA build (npm run build:web)
- Per-mode color palettes (green/cyan/amber), RDP polygon simplification
- Interactive legend, hover popup, right-click context menu, map labels
- Toast notifications, history (last 5), auto-recalculate toggle
- Status dot, error card with retry, timing feedback, copy coordinates
- Zustand: toasts, history, hiddenLayers, autoRecalculate
- Add README.md and AGENT.md

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -6,3 +6,4 @@ out
 *.log*
 .env
 *.local
+dist-web