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>

package.json

@@ -18,7 +18,8 @@
     "build:unpack": "npm run build && electron-builder --dir",
     "build:win": "npm run build && electron-builder --win",
     "build:mac": "electron-vite build && electron-builder --mac",
-    "build:linux": "electron-vite build && electron-builder --linux"
+    "build:linux": "electron-vite build && electron-builder --linux",
+    "build:web": "vite build --config vite.web.config.ts"
   },
   "dependencies": {
     "@electron-toolkit/preload": "^3.0.2",