Scheduled announcement banners

[djoufson]

Summary

• New Banner aggregate (MessageEn/Fr, optional TitleEn/Fr + SubtitleEn/Fr, Variant, StartDate/EndDate, optional CTA link, Dismissible, Priority, IsEnabled) with full CRUD through the admin panel.
• Public site renders all currently-active banners stacked at the top of MainLayout. Two layouts: a rich Microsoft-style headline + subtitle + body + pill CTA when a title is set, and a compact centered single-line layout when it isn't.
• 5 intent-based variants — Announcement, Advertisement, Promo, Event, Maintenance — each with its own gradient, icon, and contrast-aware text/CTA colors. Advertisement variant gets a subtle shine sweep.
• Banner stack pins to the top on scroll with the same threshold and slide-in animation as the navbar. A --banner-stack-height CSS custom property is computed at scroll time so the navbar slides in cleanly below the pinned banner instead of overlapping.
• Banners flagged as dismissible can be closed by visitors; the dismissal is remembered in localStorage (dismissed-banners).
• Active banners list is cached in memory for 5 minutes and invalidated on any write.

Architecture

• Domain: app.domain/Models/BannerAggregate/Banner.cs + BannerVariant enum + BannerModel view-model.
• Business: IBannerService with GetAllAsync (paged), GetAsync, GetActiveAsync, CreateAsync, UpdateAsync, DeleteAsync.
• Infrastructure: BannerService (validates EndDate > StartDate, manages cache), BannerConfigurations (indexed on IsEnabled, StartDate, EndDate), two EF migrations.
• API: AdminBannersApi mounted at /api/admin/banners (cookie auth, admin role).
• Public UI: BannerStack.razor injected at the top of MainLayout.razor + JS dismiss handler in wwwroot/js/app.js.
• Admin React: route group /admin/banners (index / new / \$id.edit), BannerForm, api/banners.ts, sidebar entry.

Test plan

• Apply the migration: dotnet ef database update --project src/app.infrastructure --startup-project src/app --context AppDbContext
• Create a banner in the admin panel without a title — verify compact centered layout renders
• Create a banner with a title + subtitle (e.g. "AI Skills Fest" / "June 8-12, 2026") — verify rich layout with bold title, uppercase subtitle, solid pill CTA
• Verify all five variants render with correct gradient, icon, and contrasting text/CTA colors
• Verify only banners where now ∈ [StartDate, EndDate] and IsEnabled = true appear on the public site
• Verify Priority (desc) controls stack order
• Dismiss a dismissible banner — verify it stays hidden on next page load (cleared via localStorage.removeItem('dismissed-banners'))
• Scroll past the navbar threshold — banner stack and navbar both pin to top; navbar sits directly under the banner with no overlap
• Scroll back up — both unpin together
• Switch language to French — banner content swaps to MessageFr / TitleFr / SubtitleFr / LinkLabelFr
• Verify the cached active-banners list refreshes within 5 min of any create/update/delete

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