fix: correct Content-Type headers for static assets on app.opencode.ai (#6113)

packages/app/public/_headers

@@ -0,0 +1,17 @@
+/assets/*.js
+  Content-Type: application/javascript
+
+/assets/*.mjs
+  Content-Type: application/javascript
+
+/assets/*.css
+  Content-Type: text/css
+
+/*.js
+  Content-Type: application/javascript
+
+/*.mjs
+  Content-Type: application/javascript
+
+/*.css
+  Content-Type: text/css