fix: align button icons and polish vendor sidebar

Fix vertical alignment of Lucide icons inside Button children across
all pages by adding verticalAlign offsets (-3px for 16px icons, -2px
for 14px icons). The design system Button wraps children in an inline
span, so SVG icons defaulted to baseline alignment.

Hide the redundant top-right "Create Tenant" button on VendorTenantsPage
when no tenants exist — the EmptyState already provides that action.

Add icons to all vendor sidebar sub-items for consistency (previously
only Email Connector had one).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/pages/tenant/TeamPage.tsx

@@ -161,7 +161,7 @@ export function TeamPage() {
       <div style={{ display: 'flex', justifyContent: 'space-between', alignItems: 'center' }}>
         <h1 style={{ margin: 0, fontSize: '1.25rem', fontWeight: 600 }}>Team</h1>
         <Button variant="primary" onClick={() => setShowInvite((v) => !v)}>
-          <Plus size={16} style={{ marginRight: 6 }} />
+          <Plus size={16} style={{ marginRight: 6, verticalAlign: -3 }} />
           Invite Member
         </Button>
       </div>
@@ -222,7 +222,7 @@ export function TeamPage() {
           description="Invite colleagues to collaborate on this tenant."
           action={
             <Button variant="primary" onClick={() => setShowInvite(true)}>
-              <Plus size={16} style={{ marginRight: 6 }} />
+              <Plus size={16} style={{ marginRight: 6, verticalAlign: -3 }} />
               Invite Member
             </Button>
           }