Move blame preferences dropdown + age legend into blob header

Context

This is a follow-up issue from #561788 (Adjust the layout of the gutter on the inline blame mode on the blob page).

Proposal

Move the blame preferences dropdown and the blame age legend into the blob header area for better discoverability and consistent placement with other blob controls.

This aligns with the design proposal in the parent issue which shows the Preview | Code | Blame toggle as part of the blob header controls.

Design Reference

current proposed
Screenshot_2026-02-12_at_10.09.56 2.2_blame-oneline__show_key

See the Figma designs linked in #561788:

Acceptance Criteria

  • Blame preferences dropdown is accessible from the blob header
  • Dropdown maintains all existing preference options
  • UI is consistent with other blob header controls
Edited by Jacques Erasmus