Wrap the top bar's buttons onto a second row when narrow (closes #272)

The top bar's two sides were held apart only by justify-between, with
no gap and no wrapping, so at 390 px "by @sneak" ended where the New
App button began. The bar's row now wraps and has a gap: in a window
too narrow for one row, New App and Logout move to a second row; on
wider windows the bar looks as before.

Model: opus-5-5

templates/base.html

@@ -26,7 +26,7 @@
 
 {{define "nav"}}
 <nav class="app-bar">
-    <div class="max-w-6xl mx-auto flex justify-between items-center">
+    <div class="max-w-6xl mx-auto flex flex-wrap justify-between items-center gap-3">
         <div class="flex items-center gap-3">
             <a href="/" class="text-xl font-medium text-gray-900 hover:text-primary-600 transition-colors">µPaaS</a>
             <span class="text-sm text-gray-500">by <a href="https://sneak.berlin" class="text-primary-600 hover:text-primary-800">@sneak</a></span>