feat(go): M6.3 — juniors page (table, filters, credits/debts, Pay buttons)

- Extract AssembleJuniors(ctx) from ServeJuniors JSON handler so HTML
  and JSON share the same view-model path (mirrors AssembleAdults pattern)
- Add JuniorsPageData wrapper in render.go
- Wire HTMLHandler.ServeJuniors to AssembleJuniors + render template
- Replace 4-line placeholder juniors.tmpl with full template:
  member table, name filter, month-range filter, totals row,
  Credits + Debts sections, Pay / Pay All buttons via /qr links
  (no Unmatched section — matches Python juniors.html parity)
- J/A attendance breakdown ("3/500 CZK (4:2J,1A)") and "?" sentinel
  rendered via MonthCell.Text from buildJuniorMemberRow, no extra
  template logic needed
- All tests pass; make parity reports 3/3 routes OK

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

go/internal/web/html_handler.go

@@ -33,7 +33,18 @@ func (h *HTMLHandler) ServeAdults(w http.ResponseWriter, r *http.Request) {
 }
 
 func (h *HTMLHandler) ServeJuniors(w http.ResponseWriter, r *http.Request) {
-	h.renderer.Render(w, "juniors", PageData{Active: "juniors", Build: h.build})
+	data, err := h.apiHandler.AssembleJuniors(r.Context())
+	if err != nil {
+		h.renderer.Render(w, "juniors", JuniorsPageData{
+			PageData: PageData{Active: "juniors", Build: h.build},
+			Error:    err.Error(),
+		})
+		return
+	}
+	h.renderer.Render(w, "juniors", JuniorsPageData{
+		PageData: PageData{Active: "juniors", Build: h.build},
+		Data:     data,
+	})
 }
 
 func (h *HTMLHandler) ServePayments(w http.ResponseWriter, r *http.Request) {