fix: enhance strong font weight in markdown rendering

[chenyme]

Summary

Fix Markdown bold rendering in chat messages and improve admin user billing edit behavior.

• Fixed Markdown **bold** / <strong> rendering so it is no longer flattened by global chat font-weight settings.
• Added explicit Streamdown strong / b component rendering with a dedicated chat strong weight.
• Adjusted chat strong weight per configured chat body weight so bold emphasis remains visible when possible.
• Fixed admin user edit save detection so subscription-only changes send the expected request instead of silently closing.
• Made admin user PATCH change detection more maintainable by treating any business payload field as a change while excluding standalone audit reason.
• Replaced custom subscription expiry calendars in user create/edit flows with the shared admin date picker.
• Added date-only support to the shared admin date picker, including localized calendar and in-popover clear action.
• Kept subscription plan and expiry date aligned in one row in the edit sheet.

Change type

• Bug fix
• Refactor

Affected areas

• Frontend / UI
• Conversations / streaming
• 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, deployment, or generated Swagger 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.