perf: optimize login first screen

[cxymds]

Summary

• Move dashboard-only API/S3/task/permission providers out of the root layout so auth routes do not load dashboard runtime.
• Defer STS/OIDC/i18n work on the login path and replace blank fallbacks with a visible loading shell.
• Add source regression tests for provider placement, lazy STS/i18n behavior, visible fallbacks, and route redirect safety.

Test Plan

• pnpm install --frozen-lockfile
• pnpm type-check
• pnpm lint
• pnpm format:check
• pnpm build
• node --test tests/lib/*.js