feat: show session title in header, allow renaming it there

[Elompenta]

Summary

• Adds the current session's title to the top bar, editable inline by clicking it.
• Reuses the existing set_session_name RPC (same one the sidebar's context-menu rename already calls).
• Moves the model dropdown and thinking-level tag to the right side of the header to make room for the title on the left.

Note on scope

pi.setSessionName() always renames whichever session pi currently has live/open - it has no concept of "rename this other historical session". Until #69 is merged, selecting an older session in the sidebar only loads it read-only without actually switching pi to it, so renaming from the header only works for the current primary/live session - the header disables the rename affordance (shows as read-only, with a tooltip) while viewing any other, historical session, rather than silently renaming the wrong one. Once #69 lands and selecting a sidebar session actually resumes it in pi, renaming from the header will work for any session you've switched to, not just the primary one.

Test plan

• Header shows the active session's name on load and updates live via the session_name broadcast (auto-title).
• Clicking the title renames the live session; sidebar entry updates to match.
• Renamed title persists across reload and across switching to a different session.
• Selecting a historical session shows its own name/preview in the header, with the rename affordance disabled.
• New session resets the header title.
• Verified on a narrow viewport that the title truncates instead of pushing the header layout.