Render the study interface while participant startup resolves

[JackWilb]

Summary

• mount the normal study routes and layout from a local non-collecting preview store as soon as the study config is parsed
• keep the entire interface inert, busy, and covered by the existing startup spinner until participant and completion state is authoritative
• pause startup-time writes, device rejection, and programmatic navigation while the barrier is active
• make participant startup idempotent across StrictMode effect replay and support safe in-place retry after recoverable initialization failures

Why

Participant startup previously withheld the complete study interface until authentication, restoration, assignment, and completion checks finished. Mounting a provisional non-interactive shell starts layout and study-resource loading earlier without exposing an incorrect task for interaction.

Validation

• yarn unittest --run (2,031 passed, 1 skipped)
• focused Shell, startup error, component controller, route, device warning, and navigation tests (92 passed)
• yarn typecheck
• yarn lint
• yarn build
• git diff --check

Browser profiling and the large-study Playwright startup scenarios remain appropriate follow-up validation before moving this draft to ready.

Closes #1320

[github-actions]

A preview of 5ac72f7 is uploaded and can be seen here:

✨ https://revisit.dev/study/PR1371 ✨

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