style+docs: consistent pre-screen styling and first-start UX docs

Setup screen shares the unlock panel look; README gains an "Erster
Start (UX)" section explaining first visit vs returning visit behaviour.

🤖 Generated with Codebuff
Co-Authored-By: Codebuff <noreply@codebuff.com>

backend/static/style.css

@@ -34,7 +34,7 @@ h1 { margin: 0 0 0.25rem; font-size: 1.6rem; }
 }
 #flash.error { color: var(--danger); }
 
-#unlock {
+#unlock, #setup {
   max-width: 420px;
   margin: 3rem auto;
   background: var(--panel);
@@ -43,10 +43,13 @@ h1 { margin: 0 0 0.25rem; font-size: 1.6rem; }
   padding: 1.5rem;
 }
 
-#unlock label { display: block; margin-bottom: 0.5rem; }
-#unlock input {
+#unlock h2, #setup h2 { margin: 0 0 0.5rem; font-size: 1.15rem; }
+.intro { color: var(--muted); font-size: 0.9rem; margin: 0 0 1rem; }
+
+#unlock label, #setup label { display: block; margin-bottom: 0.75rem; }
+#unlock input, #setup input {
   width: 100%;
-  margin-bottom: 1rem;
+  margin-top: 0.25rem;
 }
 .hint { color: var(--muted); font-size: 0.85rem; margin-top: 1rem; }