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/test/registration.test.js

@@ -214,10 +214,16 @@ it('declares its own guild slots, for core to fill', () => {
   // core page: core owns the activity feed and the forum, this module owns the
   // word "guild", so this module declares the places and core puts them in.
   //
-  // TWO slots rather than one because a slot holds one component: stacking the
-  // feed and the forum into a single fill would take away this module's ability
-  // to place them separately on its own page.
-  assert.deepEqual([...registered.declaredSlots], ['uo.guild.detail', 'uo.guild.forum'])
+  // THREE slots rather than one because a slot holds one component: stacking the
+  // feed, the forum and the notification control into a single fill would take
+  // away this module's ability to place them separately on its own page — and it
+  // does place them separately, the control above the roster and the other two
+  // below it.
+  assert.deepEqual([...registered.declaredSlots], [
+    'uo.guild.detail',
+    'uo.guild.forum',
+    'uo.guild.header',
+  ])
 })
 
 it('every declared slot is rendered by the page that owns it', () => {