feat(pwa): add proper icon sizes and update all icons from new 512px source

Replace all extension icons (16, 32, 48, 128, 192, 512) with versions
derived from the new 512x512 brand icon for consistency. Add 192x192
and 512x512 entries to manifest.webmanifest — Chrome requires both
sizes to show the "Install page as app" option in the browser menu.
Mark the 512px icon as maskable for OS launcher compatibility.

extension/dashboard/manifest.webmanifest

@@ -8,6 +8,8 @@
   "theme_color": "#0f1117",
   "icons": [
     { "src": "../icons/48.png",  "sizes": "48x48",   "type": "image/png" },
-    { "src": "../icons/128.png", "sizes": "128x128", "type": "image/png" }
+    { "src": "../icons/128.png", "sizes": "128x128", "type": "image/png" },
+    { "src": "../icons/192.png", "sizes": "192x192", "type": "image/png" },
+    { "src": "../icons/512.png", "sizes": "512x512", "type": "image/png", "purpose": "any maskable" }
   ]
 }