Split GroupServ into one file per command

REGISTER/DROP/INFO/LIST/ADD/DEL/FLAGS each move to their own file; the
two shared guards (login check, can-manage) stay in lib.rs beside the
dispatcher. No behaviour change.

modules/groupserv/src/list.rs

@@ -0,0 +1,20 @@
+use fedserv_api::{Sender, ServiceCtx, Store};
+
+// LIST: operators see every group; others see the ones they belong to.
+pub fn handle(me: &str, from: &Sender, ctx: &mut ServiceCtx, db: &mut dyn Store) {
+    let names = if from.privs.any() {
+        db.groups()
+    } else if let Some(acc) = from.account {
+        db.groups_of(acc)
+    } else {
+        Vec::new()
+    };
+    if names.is_empty() {
+        ctx.notice(me, from.uid, "No groups to show.");
+        return;
+    }
+    for n in &names {
+        ctx.notice(me, from.uid, format!("  \x02{n}\x02"));
+    }
+    ctx.notice(me, from.uid, format!("End of list ({} group(s)).", names.len()));
+}