fix(ui): stop Gantt dates shifting a day for clients behind UTC

[cavidelizade]

Summary

On the Gantt layout, dates were off by one day for anyone in a timezone behind UTC, and dragging a bar by a day saved the date it already had.

Linked issues

Closes #337

Type of change

• Bug fix (fix:)

Surface

• UI (apps/web/)

What changed

parseDay used Date.parse(input) then read local components off the resulting UTC instant, so 2026-07-16 became the 15th in the Americas. fmtDay then re-serialized from those shifted components, so a one-day drag computed 15th + 1 = 16th and saved the original date. Now parseDay takes the leading YYYY-MM-DD and builds a local-midnight date, matching how the calendar layout handles it.

Test plan

• npm run typecheck + npm run lint pass
• Manual (UTC-5 browser): a target date of 2026-07-16 now renders on the 16th, and dragging by a day saves 2026-07-17

AI assistance

• AI tools were used, tool(s): Claude Code (Opus 4.8), commits carry a Co-Authored-By: trailer

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 56243c99-932c-45ce-9e45-1996b9a521c0

📥 Commits

Reviewing files that changed from the base of the PR and between 61bc903 and d405640.

📒 Files selected for processing (1)

• apps/web/src/components/work-item/layouts/IssueLayoutGantt.tsx

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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