Fix favicon (real icon, not an unrelated orange circle) + stack encouragements with the submit button

favicon.svg was a plain solid circle, unrelated to the actual brand
mark - replaced with the real single-arch icon (matching
infrastructure/assets/icon-{light,dark}.svg), split into two
prefers-color-scheme-scoped files rather than one SVG with an embedded
@media query - the latter doesn't reliably re-evaluate via <link> on
Safari/iOS (confirmed by prior real-device testing on the Gitea side,
see gitea-head.tmpl).

Encouragements used to render right after the description, separated
from the actual submit button by the whole features/requirements
block - moved them into the same wrapper as the form, directly above
the button, so reassurance text and the call to action read together.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

public/favicon-light.svg

@@ -0,0 +1,3 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="134" height="150" viewBox="-25 -25 134 150" fill="none" stroke="#111827" stroke-width="26" stroke-linecap="round">
+  <path d="M0,100 V42 A42,42 0 0 1 84,42 V100"></path>
+</svg>