refactor: enhance AdminDateTimePicker with date granularity

[chenyme]

Summary

Fix admin user billing edits and align related settings UI behavior.

• Fixed admin user edit save detection so subscription-only changes send the expected PATCH request instead of silently closing.
• Made user PATCH change detection future-safe by treating any business payload field as a change while excluding standalone reason.
• Confirmed balance edits use the dedicated billing balance API path and are not affected by the subscription save issue.
• Replaced the account editor’s custom subscription expiry calendar with the shared admin date picker.
• Added date-only mode to the shared admin date picker, with localized calendar rendering and an in-popover clear action.
• Kept datetime mode unchanged for features that need time precision, such as redemption codes and announcements.
• Kept subscription plan and expiry date aligned in one row in the edit sheet.
• Unified user settings page content width with admin settings pages.

Change type

• Bug fix
• Refactor

Affected areas

• Frontend / UI
• Billing / payments
• Admin console

Verification

• cd frontend && pnpm lint
• git diff --check

Screenshots, API examples, or logs

Not included.

Configuration, migration, and compatibility notes

No backend API, database schema, configuration, or deployment changes.

Documentation

• Documentation is not needed for this change.

Security and privacy

• No secrets, tokens, credentials, local config, or personal data are included.
• User data access remains scoped by authenticated user context unless an admin-only path explicitly requires broader access.
• Security-sensitive behavior was reviewed for admin user updates and billing edits.

Checklist

• Changes are focused and do not include unrelated refactors.
• Tests or static verification were run where practical.
• User-facing behavior, deployment steps, API contracts, or configuration changes are documented.
• Generated artifacts are included only when this project explicitly requires them.
• Caches, build output, .pyc files, .env files, and local storage data are not committed.