fix(history): polish session list and detail UI

Rename now shows explicit Save/Cancel buttons instead of leaving
Unpin/Reveal/Delete visible mid-edit — clicking Delete while renaming
used to commit the edit and delete the session in the same frame.

All row action buttons (Pin/Unpin, Rename, Reveal, Delete, Save,
Cancel) now share one fixed size per group and the same Windows
11-style chrome as the dialog buttons, instead of each auto-sizing to
its own label — Pin/Unpin toggling no longer reflows the row, and a
narrow panel wraps the row instead of overflowing it (which was
rendering as a stray bright line at the panel boundary). The list
panel is a bit wider so the row fits on one line normally.

Also: equal-width stat tile columns in the detail view, a toned-down
synced chart cursor line, and dropped the list row's avg CPU/GPU line
since the same numbers are already in the detail view's tiles.

Refs #175