Merge Duo Chat headers into a single panel header

Nick Leonardrequested to merge
nleonard/duo-redesign-header into master

First UI slice of the Duo Chat redesign, behind the new duo_chat_redesign feature flag: the two chat header rows collapse into the single panel header.

  • Panel header shows the thread title with "GitLab Duo" (or the active agent's name) beneath, in subtle text.
  • The Orbit toggle becomes icon-only and moves into the panel header next to the session menu, via PanelActionsPortal.
  • New chats are titled "New chat", and the title updates to the goal as soon as the first prompt creates the thread.
  • Flag off keeps both header rows and current behavior.
Empty chat With a thread
header title

Screenshots include features from later MRs in the stack.

Stacked on !248639 (merged).

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