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/render.go

@@ -23,6 +23,13 @@ type AdultsPageData struct {
 	Error string
 }
 
+// JuniorsPageData is the view model for the /juniors HTML page.
+type JuniorsPageData struct {
+	PageData
+	Data  api.JuniorsResponse
+	Error string
+}
+
 // Renderer parses and executes HTML templates from the embedded FS.
 type Renderer struct {
 	tmpls map[string]*template.Template