[Due dates] Avoid false audit changes for unchanged legacy values

[Chris0Jeky]

Closes #2050

Summary

• compare the modal's calendar key with the persisted due date before constructing the update payload
• omit due-date fields for unrelated edits, including legacy non-midnight or offset-bearing values
• preserve explicit midnight-UTC date changes and clearDueDate clears

Verification

• npm ci --no-audit --no-fund
• npm test -- --maxWorkers=2 src/tests/composables/useCardModal.spec.ts
• npm run typecheck
• npx eslint src/composables/useCardModal.ts src/tests/composables/useCardModal.spec.ts
• git diff --check

Scope/docs check: no canonical docs change is required; current due-date compatibility contract remains intact.

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[Chris0Jeky]

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[Chris0Jeky]

Review gate (Codex credits exhausted, SC-9): one fresh-context reviewer confirmed the comparison is symmetric and UTC-based on both sides (toCalendarDateKey seeds the input and gates the payload), omitted fields are genuinely absent on the wire and CardService only mutates/audits the due date when DueDate.HasValue or ClearDueDate, and the spec covers offset-bearing legacy values, clear, and new-date paths with hasOwnProperty assertions. Frontend Unit green at this head. Verdict SHIP. One LOW declined (an unparseable persisted dueDate can no longer be cleared from the modal — requires hand-edited data). Merging in the next batch.