fix: eliminate two 404 sources — case mismatch + missing icons

- app.js:1195: Fix timmy_Foundation → Timmy_Foundation in vision.json API URL.
  The lowercase 't' caused a silent 404 on case-sensitive servers, preventing
  world state from loading in fetchGiteaData().

- Create icons/icon-192x192.png and icons/icon-512x512.png placeholders.
  Both manifest.json and service-worker.js referenced these but the icons/
  directory was missing, causing 404 on every page load and SW install.

Refs #707

app.js

@@ -1192,7 +1192,7 @@ async function fetchGiteaData() {
   try {
     const [issuesRes, stateRes] = await Promise.all([
       fetch('https://forge.alexanderwhitestone.com/api/v1/repos/Timmy_Foundation/the-nexus/issues?state=all&limit=20'),
-      fetch('https://forge.alexanderwhitestone.com/api/v1/repos/timmy_Foundation/the-nexus/contents/vision.json')
+      fetch('https://forge.alexanderwhitestone.com/api/v1/repos/Timmy_Foundation/the-nexus/contents/vision.json')
     ]);
 
     if (issuesRes.ok) {