Extract pure helpers from LetterStructure.js and test actual production code

[Copilot]

• Analyze CI failure: npm ci failing with "Missing from lock file" errors
• Root cause: previous session ran npm install which upgraded package-lock.json and app/package-lock.json from lockfileVersion 1 to lockfileVersion 3
• Restore both lock files to their original lockfileVersion 1 content (matching feature/add-tests base branch)
• Restore deleted getCurrentContent structure tests to letterStructure.test.js
• Confirm all tests pass (47 passing)

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[Samuel3]

@copilot Please fix broken build

[Copilot]

@copilot Please fix broken build

Fixed in commit d544b68. The CI failure was caused by accidentally committing updated package-lock.json files — a previous npm install run upgraded them from lockfileVersion 1 to lockfileVersion 3, which caused npm ci to report packages as missing. Both package-lock.json and app/package-lock.json have been restored to their original lockfileVersion 1 content (matching the feature/add-tests base branch), since the code changes don't require any new packages.