DC-456 - spinner-only loading state for DC id-proofing; stop copy key leak

[necampanini]

🔗 Jira ticket

DC-456

✍️ Description

DC users who submitted the id-proofing form briefly saw the literal text "title" and "body" inside a blue box while the request was in flight. The form used LoadingInterstitial with i18n keys from the step-upProcessing namespace, which is only populated for CO (DC's content sheet marks those rows !N/A!), so i18next fell back to the key names. The loading branch now gates on copy presence in the active locale bundle (i18n.exists('step-upProcessing:title') && i18n.exists('step-upProcessing:body')): when the keys exist the titled LoadingInterstitial renders, when they don't the form falls back to a centered USWDS spinner inside a role="status" region. Adding the copy upstream is enough to switch in the titled interstitial for any state, no code change needed.

🔗 Links to related PRs

• {sibling repo / branch — leave a placeholder if not yet pushed}

✅ Completion tasks

• Added relevant tests
• Meets acceptance criteria in ticket
• Configuration changes:
  • If new environment variables are added, update in Tofu
  • If new environment secrets are added, update in Tofu and set in AWS Secret Manager
  • If you're adding an appsetting, add it to the requisite example file, and update it in the AWS AppConfig
  • If appsetttings are changed, update in AWS AppConfig

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Priority Review Table

Priority	File	Unit	Sec	Cpx	Nov	Notes
🟡	IdProofingForm.tsx	loading-state branch	Low	Low	Med	new presence-check gate, a11y on spinner
🟢	IdProofingForm.test.tsx	two new tests covering both gate branches	Low	Low	Low	i18n bundle add/remove via try/finally