feat(guilds): a third place on the guild page, and where that page lives

Two lines only core cannot supply for itself.

`uo.guild.header` is a third declared slot, at the top of the page, for core's
per-Team notification control. A third rather than a corner of the feed because a
slot holds one component and the first fill wins: the control is an action ON this
page and the other two are content IN it, and separate slots are what let this
module say so.

`pageUrlTemplate` tells core where a guild page actually is. Teams are a contract
primitive with no core surface — core owns the tables and the access rules, this
module owns the word "guild" and therefore the page — which leaves core unable to
write a link to one. A notification email that cannot take you to the thread it is
about is most of the way to useless. Core substitutes `{externalId}` and does
nothing else with it; a template naming its own host is refused at registration.

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

client/src/routes/public/Guild.jsx

@@ -75,6 +75,13 @@ export default function Guild() {
               {data.alliance && ` · ${data.alliance}`}
             </p>
 
+            {/* A third place for core, up here rather than below the roster: core
+                puts this guild's notification control in it, and a control that
+                acts on the page belongs beside the page's title and not after its
+                content. Empty for a visitor with no membership, and on a core
+                that fills nothing. */}
+            <Slot name="uo.guild.header" externalId={String(id)} moduleId="uo" />
+
             {roster.length > 0 && (
               <div style={{ overflowX: 'auto', marginTop: 18 }}>
                 <table style={{ width: '100%', borderCollapse: 'collapse' }}>