feat(ui): structured event detail (stack trace, breadcrumbs, tags, request, user) #339

Event detail page (web/src/routes/(dashboard)/issues/[issueId]/events/[eventId]/+page.svelte:137) renders only JSON.stringify(event.data) in a <pre>.

Render structured panels: exception with frames (in-app highlighted, library frames collapsed), breadcrumbs timeline, tags as chips, request, user/device/OS/release. Keep a 'Raw JSON' tab as fallback. Add prev/next event navigation and 'latest event' default.

Found in UI review of develop @ db3ca67.