[Capture][a11y] Fix diagnostics mock and receipt announcement replay

[Chris0Jeky]

Closes #2147

Summary

• complete the integration test error-mapper mock and pin API error propagation
• announce a failed capture receipt with role=alert once, then keep it visible without replay when revisited
• preserve the existing draft, error association, and per-variant behavior

Verification

• npm ci --no-audit --no-fund
• npm test -- --maxWorkers=2 src/tests/store/captureStore.integration.spec.ts src/tests/views/paper/PaperInboxView.spec.ts
• npm run typecheck
• npx eslint src/views/paper/PaperInboxView.vue src/tests/store/captureStore.integration.spec.ts src/tests/views/paper/PaperInboxView.spec.ts
• git diff --check

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[Chris0Jeky]

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[Chris0Jeky]

Review gate (Codex credits exhausted, SC-9): one fresh-context reviewer confirmed the acknowledged flag is per-variant and reset on every new failure (so a later failure re-announces), the retry path unmounts and remounts the alert node, the success role=status regions are untouched, and the mock completion strengthens the store assertion to rethrow the original API error. Verdict SHIP. Three LOWs declined as non-blocking, noted for #2147's owner: no spec asserts the re-announce half (a second mockRejectedValueOnce + role===alert would close it); captureErrorAcknowledged is not cleared when the error clears (harmless today); the remount ordering is reasoned, not executed.