fix(web): explain unavailable OAuth integrations

[daniel-lxs]

Summary

• detect whether deployment-managed OAuth credentials are ready without exposing credential names or values
• explain missing or incomplete Linear setup directly on the integration card
• guide admins to the Linear setup documentation and non-admins to contact an administrator
• keep existing Linear installations manageable and surface useful server errors

In simple words

Roomote now checks whether Linear can actually be connected before showing the Enable action. If the deployment is missing part or all of its OAuth setup, users see what is wrong instead of entering a flow that can only fail.

Validation

• manually verified the signed-in local integrations page and existing Linear installation flow in Brave
• 54 focused Vitest tests
• web lint and TypeScript checks
• full pre-push oxlint, residual lint, fast typecheck, and Knip gates

[roomote-roomote]

No code issues found. See task

_{Reviewed 0f9d625}