Web UI: Jarl logo and wordmark

Add the logo, a Viking jarl drawn like a woodcut whose spear-pointed crown
holds a DNS tree. It is generated by scripts/logo.py, which mirrors the
left half so the figure is exactly symmetrical, as logo.svg (white on a
Nordic-blue disc) and logo-white.svg. It links home in the header, sits on
the login page and is the favicon.

The login page shows an angular "JARL" wordmark drawn as SVG paths over a
rule with a "DNS" node, coloured from the theme. The header's "Jarl" uses
a self-hosted Cinzel Bold subset (SIL OFL); jarld serves .woff2 as
font/woff2.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -1,3 +1,5 @@
+<p align="center"><img src="internal/web/static/assets/logo.svg" width="120" alt="Jarl logo: a Viking jarl in a spear-pointed crown holding a DNS tree"></p>
+
 # Jarl
 
 *A Jarl rules a Hold; this one rules your zones.*
@@ -139,3 +141,12 @@ review step.
 Version 0.1.0, the first release. It is feature-complete for one server's
 configuration. DNSSEC signing and managing several servers are not handled
 yet ([concepts.md](docs/concepts.md#what-jarl-deliberately-doesnt-do-yet)).
+
+## Credits
+
+The "Jarl" name in the web UI's header uses [Cinzel](https://github.com/NDISCOVER/Cinzel) Bold
+by the Cinzel Project Authors, under the SIL Open Font License 1.1
+(`internal/web/static/assets/fonts/Cinzel-OFL.txt`). The logo
+(`assets/logo.svg`, and `assets/logo-white.svg` for dark backgrounds) is part
+of Jarl: a Viking ruler drawn like a woodcut, whose crown holds a DNS tree
+(a root node with its branches). It is generated by `scripts/logo.py`.