fix(web): keep workspace chooser transition stable

[setkyar]

Summary

• keep the workspace chooser in the main web bundle so in-app navigation does not pause on a lazy-route fallback
• show a chooser-shaped loading skeleton while auth and workspace membership restore
• avoid displaying the auth/onboarding loading surface before /workspaces
• add regression coverage for the workspace chooser loading state

Root cause

ProtectedRoute reused the auth/onboarding loading screen for chooser-mode routes while session state restored. The workspace chooser was also lazy-loaded, adding a second fallback during navigation. Together these made switching workspaces look like a brief return to login.

Validation

• bun test apps/web/src — 724 passed
• bunx biome check ...
• bun run typecheck
• bun run --filter @wateaminbox/web build
• git diff --check

Build warnings are limited to the existing lottie-web eval and chunk-size warnings.