Add a support inbox icon to the topbar (sow-nodebb-plugin-support#10)

Mirrors the existing unread-drawer pattern: a new dropdown next to the
notification bell, gated on templateData.support.visible (injected by
the support plugin's filter:middleware.renderHeader hook, on every page
render, for viewers with any support access at all). Opening it fetches
GET /plugins/support/notifications via the api module, renders up to
10 entries by textContent (never innerHTML), and marks everything read
via POST .../notifications/read, clearing the badge client-side.

Also present in the mobile burger drawer, same gating.

ponytail: no live socket push for this badge (core's own unread/notification
counts get one); it only catches up on the next full page load after the
box is opened. Noted in client.js as the upgrade path if that gap matters.

Verified live against sow-nodebb-plugin-support's dev NodeBB stack (theme +
plugin both mounted): icon hidden for a user with no support access, badge
count and dropdown contents correct for a reviewer, badge clears on open.
Full plain-node test suite passes.

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

tests/global-topbar-contract.test.js

@@ -180,3 +180,39 @@ assertIncludes(
 	'.unread-dropdown',
 	'Topbar styles should cover the unread dropdown'
 );
+
+const supportDrawer = read('templates/partials/header/support-drawer.tpl');
+assertIncludes(
+	topbar,
+	'<!-- IMPORT partials/header/support-drawer.tpl -->',
+	'Topbar should mount the support inbox drawer'
+);
+assertIncludes(
+	topbar,
+	'{{{ if support.visible }}}',
+	'Support inbox drawer should be gated on the viewer having any support access'
+);
+assert(
+	topbar.indexOf('partials/sidebar/notifications.tpl') < topbar.indexOf('partials/header/support-drawer.tpl'),
+	'Support inbox should sit next to (after) the notifications bell'
+);
+assertIncludes(
+	supportDrawer,
+	'component="support/count"',
+	'Support toggle should carry its own unread count badge'
+);
+assertIncludes(
+	supportDrawer,
+	'data-wg-support-menu',
+	'Support dropdown menu should expose the JS mount hook'
+);
+assertMatches(
+	topbar,
+	/href="\{relative_path\}\/support"[^>]*>[\s\S]*?component="support\/count"/,
+	'Mobile drawer actions should include a Support inbox link with count badge'
+);
+assertIncludes(
+	stylesheet,
+	'.support-dropdown',
+	'Topbar styles should cover the support inbox dropdown'
+);