The profile-switch skeleton (#4671) always rendered a fixed content shape; for
a brand-new/empty profile that flashed a misleading 'loading conversations'
skeleton that resolved to an empty list. Track each profile's last-seen session
count (localStorage, per profile name, recorded after each non-all-profiles
render) and, when switching into a profile KNOWN to have zero sessions, render a
quiet empty-state placeholder instead of the content skeleton. Unknown profiles
keep the content skeleton (safe default); skipped under an active project/source
filter (the count is an unfiltered total). prefers-reduced-motion + light/dark
aware. Frontend-only. 7 source-structural tests; Phase-1 skeleton tests still
green.
