feat(pwa): installable PWA + responsive mobile dashboard

- Add Web App Manifest + ArbPulse icons (192/512/maskable/apple-touch) under
  web/public/; no service worker (real-time honesty: never serve stale data).
- Add mobile/install metadata to index.html (theme-color, viewport-fit=cover,
  apple-mobile-web-app-*, manifest + apple-touch-icon links).
- Responsive layout: PriceMatrix stacked per-venue cards below sm, StatsBar
  reflow, ~44px touch targets in Controls/ConfigPanel, safe-area top inset.
- scripts/gen-icons.ps1 derives icon sizes from the master (no new deps).
- openspec: add-pwa-mobile change (proposal/design/specs/tasks).

Co-authored-by: Cursor <cursoragent@cursor.com>

web/public/manifest.webmanifest

@@ -0,0 +1,31 @@
+{
+  "name": "Arb Pulse — BTC Arbitrage Engine",
+  "short_name": "Arb Pulse",
+  "description": "Real-time cross-exchange BTC arbitrage detection & simulation (Kraken, Bybit, OKX, Binance).",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "orientation": "any",
+  "theme_color": "#0a0e14",
+  "background_color": "#0a0e14",
+  "icons": [
+    {
+      "src": "/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/icon-maskable-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "maskable"
+    }
+  ]
+}