M1 progress. Adds the post-login shell: HeaderBar (nav tabs and
search), Sidebar (playlists list and user card), PlayerBar (transport,
now-playing, volume), and AppShell composing them with the content
region as a placeholder.
app.slint gains an AppScreen enum; clicking sign-in or start-offline
navigates login -> shell (no real auth yet — that is M2). New layout
foundation tokens (sidebar width, header/player-bar heights) plus
border and hover semantic colors.
cc
