Add a Settings > Email previews page showing every branded email with sample data

Renders the real templates every send function uses (claim approved,
membership invoice, invoice reminder) against hand-built placeholder
context -- no real Member/ClubMembership/DuesInvoice row needed, so
nothing here can leak real data. Each card shows the subject line and
toggles between the HTML render (in an iframe, so the email's own
markup can't clash with the page's) and the plain-text body, using
the same bg-steel pill-toggle pattern as the news preview's NL/EN
switch (generalised to .view-toggle-btn, kept .news-lang-btn as an
alias). Admin-only, alongside Club identity in Settings.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ECGMEwrc2k4D8VQuwjstj9

management/context_processors.py

@@ -127,6 +127,7 @@ _NAV_SECTIONS = {
     "form_list": "form_list",
     "submission_list": "form_list",
     "club_settings": "club_settings",
+    "email_preview_list": "email_preview_list",
     "onboarding_requirement_list": "onboarding_requirement_list",
     "onboarding_requirement_create": "onboarding_requirement_list",
     "onboarding_requirement_update": "onboarding_requirement_list",
@@ -172,6 +173,7 @@ _TOP_SECTION = {
     "invoice_list": "finance",
     "form_list": "settings",
     "club_settings": "settings",
+    "email_preview_list": "settings",
     "onboarding_requirement_list": "settings",
     "role_list": "settings",
     "position_list": "settings",