v0.167.1: center the sidebar logo (margin: 0 auto)

.sidebar-logo was left-aligned; add auto side-margins so the 140px logo centers in
the header (desktop sidebar + mobile drawer share the element). Adds TestSidebarLogo_Centered.

controller/internal/web/mobile_nav_test.go

@@ -88,6 +88,24 @@ func TestMobileCSS_StripRemovedDrawerPresent(t *testing.T) {
 	}
 }
 
+// The sidebar logo is horizontally centered in its header (desktop sidebar + mobile drawer share
+// the element). Red-proof: revert to `margin-bottom: 0.25rem` (no auto side-margins) → this FAILS.
+func TestSidebarLogo_Centered(t *testing.T) {
+	data, err := StyleCSS()
+	if err != nil {
+		t.Fatalf("StyleCSS(): %v", err)
+	}
+	css := string(data)
+	i := strings.Index(css, ".sidebar-logo {")
+	if i < 0 {
+		t.Fatal(".sidebar-logo rule not found")
+	}
+	rule := css[i : i+strings.Index(css[i:], "}")]
+	if !strings.Contains(rule, "margin: 0 auto") {
+		t.Errorf(".sidebar-logo is not centered (no `margin: 0 auto`); rule was:\n%s", rule)
+	}
+}
+
 // Group A — drawer markup (Scenario A/C), asserted through the real layout render.
 //
 // Red-proof: remove aria-controls from the burger button in layout.html → the aria-controls