Epic: Resolve UI inconsistencies and consolidation

Track and resolve the UI inconsistencies, dark mode gaps, and consolidation opportunities identified in the [frontend UI audit (#6606)](https://github.com/Flagsmith/flagsmith/issues/6606).

Full findings in the [audit report](https://github.com/Flagsmith/flagsmith/issues/6606#issuecomment-4025903243). Detailed documentation in [`frontend/DESIGN_SYSTEM_ISSUES.md`](https://github.com/Flagsmith/flagsmith/blob/chore/design-system-audit-6606/frontend/DESIGN_SYSTEM_ISSUES.md).

## Related

- [#5746](https://github.com/Flagsmith/flagsmith/issues/5746) — TS migration epic
- [#5921](https://github.com/Flagsmith/flagsmith/issues/5921) — UI improvements
- [PR #6764](https://github.com/Flagsmith/flagsmith/pull/6764) — React 19 migration
- Audit branch: `chore/design-system-audit-6606`
- Tokens branch: `chore/design-system-tokens`
- [Penpot visual inventory](https://design.penpot.app/#/workspace?team-id=72ad1239-4f5c-8115-8007-a72d5b669ca5&file-id=72ad1239-4f5c-8115-8007-a72dd0aef2fe&page-id=72ad1239-4f5c-8115-8007-a72dd0aef2ff)