docs: put the Cairn OBS logo at the top of the README

Uses the brand assets already in the repo rather than adding a copy --
web/src/lib/assets/logo-horizontal-{light,dark}.svg are the same files
NavSidebar renders, so the README cannot drift from the app's logo.

Wrapped in <picture> with a prefers-color-scheme source, since the two
variants differ by the colour they are drawn *for*, not by name: -light
carries #111315 text for light backgrounds, -dark carries #F2F2F2 for
dark ones. A single file would have been near-invisible on one of
GitHub's two themes. Both have transparent backgrounds -- the only
<rect> in each is a small design element, not a fill -- so neither
variant shows a card behind it.

README.md

@@ -1,3 +1,12 @@
+<p align="center">
+  <picture>
+    <source media="(prefers-color-scheme: dark)"
+            srcset="web/src/lib/assets/logo-horizontal-dark.svg">
+    <img src="web/src/lib/assets/logo-horizontal-light.svg"
+         alt="Cairn OBS" width="420">
+  </picture>
+</p>
+
 # Cairn OBS
 
 Open-core, Kubernetes-native log aggregation and observability. Built to match