Dashboard: unify Most used tags and Most active authors chip colors

[tystar86]

Summary

Most used tags chips use a 3-color cycle (orange --sage, grey/neutral --rose, light --bg). Most active authors only alternates two colors (--bg and --sage), so the panels look inconsistent.

Expected

Both sections should use the same orange / white / grey rotation (mapped to existing theme variables).

Implementation notes

• frontend/src/pages/Dashboard.jsx: share one helper or the same i % 3 mapping for most_used_tags and top_authors --chip-bg.

Linear

https://linear.app/tystar/issue/TYS-186/dashboard-align-tag-and-author-chip-colors-orange-white-grey

[tystar86]

Cross-linked from Linear: https://linear.app/tystar/issue/TYS-186/dashboard-align-tag-and-author-chip-colors-orange-white-grey