feat(go): M6.5 — member-detail modal JS module for /adults and /juniors

Adds static/js/member-detail.js: fetches /api/<page> once on page load,
caches the response, and renders a per-member detail modal on [i] row click.
Keyboard nav: Esc closes, ↑/↓ walk visible (filtered) rows. All modal CSS
was already in place from M6.1.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

go/internal/web/html_handler_test.go

@@ -127,6 +127,47 @@ func TestAdultsPage(t *testing.T) {
 	}
 }
 
+func TestModalMarkup(t *testing.T) {
+	renderer, err := web.NewRenderer()
+	if err != nil {
+		t.Fatalf("NewRenderer: %v", err)
+	}
+	b := web.BuildInfo{Version: "v0", Commit: "abc1234", BuildDate: "2026-01-01"}
+	h := web.NewHTMLHandler(renderer, b, fixtureHandler(t))
+
+	cases := []struct {
+		path    string
+		handler http.HandlerFunc
+		page    string
+		wantRow string // data-name present only when the page has rows
+	}{
+		{"/adults", h.ServeAdults, "adults", `data-name="Test Member"`},
+		{"/juniors", h.ServeJuniors, "juniors", ""},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.path, func(t *testing.T) {
+			req := httptest.NewRequest(http.MethodGet, tc.path, nil)
+			w := httptest.NewRecorder()
+			tc.handler(w, req)
+			body := w.Body.String()
+
+			for _, want := range []string{
+				`data-page="` + tc.page + `"`,
+				`id="memberModal"`,
+				`/static/js/member-detail.js`,
+			} {
+				if !strings.Contains(body, want) {
+					t.Errorf("body missing %q", want)
+				}
+			}
+			if tc.wantRow != "" && !strings.Contains(body, tc.wantRow) {
+				t.Errorf("body missing info-icon row %q", tc.wantRow)
+			}
+		})
+	}
+}
+
 func TestPaymentsPage(t *testing.T) {
 	renderer, err := web.NewRenderer()
 	if err != nil {