feat(web): wire toasts through Sonner on discover & organizers pages (#779)

[Rufai-Ahmed]

Summary

Routes the remaining ad-hoc notifications through the global Sonner provider, per #779.

Sonner is already installed and <Toaster /> is mounted in apps/web/app/layout.tsx, and create-event / TicketModal already use toast. However the discover and organizers pages each reimplemented their own toast (local toastMessage state + setTimeout + a hand-rolled fixed div) instead of using it. This PR replaces those with toast.error() so all pages share one consistent, accessible toast system.

Changes

• app/discover/page.tsx & app/organizers/page.tsx: drop the ad-hoc toastMessage state and rendered <div>; showErrorToast now calls toast.error(message) from sonner (removed the now-unused useState import in organizers).
• app/layout.tsx: set <Toaster position="bottom-right" /> to match the acceptance criteria (was top-right), and added a usage doc-comment at the top of the file.

Acceptance criteria

• ✅ Toasts now render bottom-right across all pages via the single global <Toaster />.
• ✅ The discover/organizers error callbacks go through toast.error instead of a separate, easily-missed local banner.

Note

I changed the Toaster position from top-right to bottom-right to match the issue's acceptance criteria — happy to revert if top-right was intentional. Couldn't run a full next build in my environment (Next's SWC binary download was blocked), but the changes mirror the existing toast usage already working in create-event/TicketModal.

Closes #779

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