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/api/handler.go

@@ -55,14 +55,23 @@ func (h *Handler) AssembleAdults(ctx context.Context) (AdultsResponse, error) {
 
 // ServeJuniors handles GET /api/juniors.
 func (h *Handler) ServeJuniors(w http.ResponseWriter, r *http.Request) {
-	ctx := r.Context()
-	members, sortedMonths, txns, exceptions, err := h.loadAll(ctx, false)
+	resp, err := h.AssembleJuniors(r.Context())
 	if err != nil {
 		h.writeError(w, r, err)
 		return
 	}
+	writeJSON(w, resp)
+}
+
+// AssembleJuniors loads all data and builds the juniors view model.
+// Shared between the JSON API route and the HTML handler.
+func (h *Handler) AssembleJuniors(ctx context.Context) (JuniorsResponse, error) {
+	members, sortedMonths, txns, exceptions, err := h.loadAll(ctx, false)
+	if err != nil {
+		return JuniorsResponse{}, err
+	}
 	result := domreconcile.Reconcile(members, sortedMonths, txns, exceptions, time.Now().Year())
-	writeJSON(w, buildJuniorsResponse(members, sortedMonths, result, txns, h.Config, time.Now().Format("2006-01")))
+	return buildJuniorsResponse(members, sortedMonths, result, txns, h.Config, time.Now().Format("2006-01")), nil
 }
 
 // ServePayments handles GET /api/payments.