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>

vite.web.config.ts

@@ -0,0 +1,15 @@
+import { resolve } from 'path'
+import { defineConfig } from 'vite'
+import react from '@vitejs/plugin-react'
+
+export default defineConfig({
+  root: 'src/renderer',
+  plugins: [react()],
+  resolve: {
+    alias: { '@renderer': resolve('src/renderer/src') }
+  },
+  build: {
+    outDir: '../../dist-web',
+    emptyOutDir: true
+  }
+})