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/vendor/CertificatesPage.tsx

@@ -225,7 +225,7 @@ export function CertificatesPage() {
                   onClick={handleActivate}
                   loading={activateMutation.isPending}
                 >
-                  <ShieldCheck size={14} style={{ marginRight: 6 }} />
+                  <ShieldCheck size={14} style={{ marginRight: 6, verticalAlign: -2 }} />
                   Activate
                 </Button>
                 <Button
@@ -233,7 +233,7 @@ export function CertificatesPage() {
                   onClick={handleDiscard}
                   loading={discardMutation.isPending}
                 >
-                  <Trash2 size={14} style={{ marginRight: 6 }} />
+                  <Trash2 size={14} style={{ marginRight: 6, verticalAlign: -2 }} />
                   Discard
                 </Button>
               </>
@@ -252,7 +252,7 @@ export function CertificatesPage() {
                 loading={restoreMutation.isPending}
                 disabled={expired}
               >
-                <RotateCcw size={14} style={{ marginRight: 6 }} />
+                <RotateCcw size={14} style={{ marginRight: 6, verticalAlign: -2 }} />
                 {expired ? 'Expired' : 'Restore'}
               </Button>
             }
@@ -284,7 +284,7 @@ export function CertificatesPage() {
               onClick={handleUpload}
               loading={stageMutation.isPending}
             >
-              <Upload size={14} style={{ marginRight: 6 }} />
+              <Upload size={14} style={{ marginRight: 6, verticalAlign: -2 }} />
               Stage Certificate
             </Button>
           </div>