Exit nodes: block IPv6 while one is in use; exit icon on the network tile

IPv6 has no way through an exit node, so the same policy rules get an
unreachable default for IPv6 instead of leaking it. The tray shows a green
icon in the tile corner while an exit node is in use, with a one-line
confirmation to stop it; confirmations are shortened.

Co-Authored-By: Claude <noreply@anthropic.com>

crates/tsunagi-cli/src/main.rs

@@ -3385,7 +3385,8 @@ fn exit_rows(network: &tsunagi::ipc::NetworkReport) -> Vec<report::Row> {
                     Health::Good,
                     "using exit",
                     format!("{name}  ·  all internet traffic leaves through it"),
-                ),
+                )
+                .with_note("IPv6 is blocked meanwhile, so it cannot leave the ordinary way"),
                 Some(rules) => Row::new(
                     Health::Degraded,
                     "using exit",