Fix Coding page loading for participants without transcripts

[JackWilb]

Summary

• show a participant-level empty state when Coding has no component or transcript data
• keep the Coding footer and participant navigation available in that state
• only mount the waveform and show its loading overlay when the selected Participant actually has the routed task
• cover absent and stale task IDs, genuine task loading, and participant switching with regression tests

Root cause

The waveform loading state started as active even when there was no task to mount. A stale task ID could also be treated as loadable after switching to a Participant whose answers did not contain that task, allowing the footer overlay to obscure participant navigation indefinitely.

Validation

• yarn unittest --run src/analysis/individualStudy/thinkAloud/tests/ThinkAloudAnalysis.spec.tsx src/components/audioAnalysis/tests/AudioProvenanceVis.spec.tsx — 90 passed
• yarn unittest --run — 2,032 passed, 1 skipped
• yarn typecheck
• yarn lint
• yarn build
• git diff --check
• independent adversarial review completed; both reported stale-route and test-coverage findings were fixed and revalidated

Fixes #1335

[github-actions]

A preview of is uploaded and can be seen here:

✨ https://revisit.dev/study/PR1372 ✨

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