fix: unify table scrolling and Firefox column sizing

[chenyme]

Summary

Fix inconsistent table rendering and scrollbar behavior across Firefox and Chromium browsers.

• Move table scrolling into the shared Table component using Radix overlay scrollbars.
• Define stable column widths for the subscription usage log to prevent Firefox from collapsing the table to its first column.
• Remove obsolete native scrollbar height compensation.
• Remove redundant page-level overflow declarations from virtual tables, billing pricing, group access, and upstream model synchronization.
• Preserve dynamic table height transitions, sticky headers, virtualization, and overscroll behavior.

Change type

• Bug fix
• Refactor
• Feature
• Documentation
• Configuration / deployment
• Security hardening
• Other

Affected areas

• Frontend / UI
• Backend / API
• Authentication / authorization
• Conversations / streaming
• Files / RAG / extraction
• Model routing / providers
• MCP / tools
• Billing / payments
• Admin console
• Deployment / Docker / configuration
• Documentation

Verification

• pnpm check
• pnpm build
• git diff --check

Screenshots, API examples, or logs

Before:

• Firefox could render only the first subscription usage log column.
• Native scrollbar gutters left visible white areas on the right and bottom edges of tables.

After:

• Table columns retain deterministic widths across supported browsers.
• Scrollbars are rendered as shared overlay controls without consuming table layout space.

Configuration, migration, and compatibility notes

No configuration, database, API contract, or migration changes are required.

The change keeps existing table virtualization, sticky headers, dynamic height transitions, and touch scrolling behavior.

Documentation

• Documentation is not needed for this change.
• Documentation was updated.
• Documentation still needs to be updated.

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, including authentication, authorization, provider routing, file processing, billing, and admin APIs where relevant.

Checklist

• I searched existing issues and pull requests.
• 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.