fix(web): use backend URL fallback for direct fetches

[GautamSharma99]

This fixes several direct web fetches that could build invalid undefined/... URLs when
NEXT_PUBLIC_BACKEND_URL is not configured.

Most of the app already falls back to https://api.supermemory.ai, but a few direct
fetch/router calls skipped that fallback. This PR adds a shared getBackendUrl() helper and
uses it for affected web requests.

Changes

• Add getBackendUrl() in apps/web/lib/url-helpers.ts

• Normalize trailing slashes from configured backend URLs

• Use the fallback URL for:

  • OTP magic-link verification redirect
  • chat thread list/load/delete requests
  • space highlights requests and cache key
  • memory-of-day request
  • file upload request

• Add tests for default fallback, custom URL, and trailing slash handling

Testing

• bun test apps/web/lib/url-helpers.test.ts

• bunx biome check apps/web/lib/url-helpers.ts apps/web/lib/url-helpers.test.ts apps/web/
  components/chat/index.tsx apps/web/components/app-experience.tsx apps/web/hooks/use-
  document-mutations.ts 'apps/web/app/(auth)/login/page.tsx'

• Verified no remaining unsafe direct interpolations with:

  • rg "NEXT_PUBLIC_BACKEND_URL\}/" -n apps/web