feat: proper logo, favicon, and pwa icons

- replace placeholder house svg with clean geometric house mark
  on indigo-700 (#4338CA) — roof triangle, body, door cutout
- add favicon.svg with prefers-color-scheme dark variant
- add icon-16/32 sizes; regenerate all pngs from svg via sharp
- update BrandMark component to use inline svg house instead of 'f'
- wire favicon + sized pngs into layout metadata
- fix middleware to allow icon/manifest/favicon paths without auth
  (android launcher fetches icons in a cookieless system context)
- fix getCurrentSession() to redirect to /login instead of throwing
  when session cookie is stale/invalid

public/favicon.svg

@@ -0,0 +1,9 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512" width="512" height="512">
+  <style>
+    @media (prefers-color-scheme: dark) { .bg { fill: #6366F1; } }
+  </style>
+  <rect class="bg" width="512" height="512" rx="96" fill="#4338CA"/>
+  <rect x="130" y="238" width="252" height="190" fill="white"/>
+  <polygon points="256,82 68,254 444,254" fill="white"/>
+  <rect x="216" y="338" width="80" height="90" rx="8" fill="#4338CA"/>
+</svg>