Add Svelte error page handling

[ejsmith]

Adds a shared Svelte error page for routed failures and makes missing saved-view slugs return a real 404 instead of rendering the Events or Stacks list pages.

Summary:

• Add a status-aware +error.svelte with recovery links for authenticated and unauthenticated users.
• Add client-side saved-view slug route guards for Events and Stacks routes.
• Surface missing saved-view slugs through the shared 404 page.
• Add regression coverage for missing saved-view slug detection.

Validation:

• npm run format
• npm run test:unit -- src/lib/features/saved-views/use-saved-views.test.ts
• npm run check
• npm run lint
• npm run build
• Browser verified /next/stack/most-frequent-404s2 and /next/event/this-view-does-not-exist render the 404 page.

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
Exceptionless.Insulation	25%	23%	203	❌
Exceptionless.Web	73%	62%	3938	➖
Exceptionless.AppHost	18%	9%	82	❌
Exceptionless.Core	69%	63%	7903	➖
Summary	68% (13674 / 20010)	62% (7197 / 11672)	12126	➖