Adds the Network button to the artist action cluster (Heart / Build
collection / Radio / Network), wired to NetworkSidebarState.open.
While the sidebar is open the button stays "active" — accent-tinted
icon over the same surface a hover would produce — mirroring Tauri's
`.network-btn.active`. The CircleAction primitive grew an `active`
prop to express this without bespoke per-button styling; existing
buttons default it to false and are unaffected.
The click toggles NetworkSidebarState.open and emits
NetworkSidebarActions.toggle(). Rust persists the new flag through
network_sidebar_prefs::set_open() so the sidebar reopens in the same
state the user left it.
Asset: element-connect.svg copied from the Tauri `static/` folder
verbatim — same icon Tauri renders for the button.
Hide (blacklist) button stays out of the cluster until the
artist_blacklist module migrates out of src-tauri. The sidebar
itself, its content, and the controllers that fill the state are
landed in the following phases.
cc
vicrodh
a8c8947658