feat(frontend): use reference icons for PWA manifest and favicons

- Copy all icon PNGs from docs/reference (android, apple, ms, favicon sizes)
- Copy favicon.ico and browserconfig.xml
- Manifest: full icon set (36–310px), background/theme #312F45
- app.html: favicon links, full apple-touch-icon set, MS tile metas

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

frontend/static/browserconfig.xml

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="utf-8"?>
+<browserconfig>
+    <msapplication>
+        <tile>
+            <square70x70logo src="/images/ms-icon-70x70.png" />
+            <square150x150logo src="/images/ms-icon-150x150.png" />
+            <square310x310logo src="/images/ms-icon-310x310.png" />
+            <TileColor>#615880</TileColor>
+        </tile>
+    </msapplication>
+</browserconfig>