Add SaaS frontend code (#5879)

# Description of Changes
Adds the code for the SaaS frontend as proprietary code to the OSS repo.
This version of the code is adapted from 22/1/2026, which was the last
SaaS version based on the 'V2' design. This will move us closer to being
able to have the OSS products understand whether the user has a SaaS
account, and provide the correct UI in those cases.

frontend/src/saas/constants/authProviders.ts

@@ -0,0 +1,13 @@
+export type OAuthProviderId = 'google' | 'apple' | 'github' | 'azure'
+
+export type OAuthProviderMeta = {
+  id: OAuthProviderId
+  label: string
+  file: string
+  isDisabled: boolean
+}
+
+export const oauthProviders: readonly OAuthProviderMeta[] = [
+  { id: 'google', label: 'Google', file: 'google.svg', isDisabled: false },
+  { id: 'github', label: 'GitHub', file: 'github.svg', isDisabled: false },
+] as const