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/register.rs

@@ -0,0 +1,20 @@
+use fedserv_api::{Sender, ServiceCtx, Store};
+
+// REGISTER <!group>: create a group with you as its founder.
+pub fn handle(me: &str, from: &Sender, name: Option<&str>, ctx: &mut ServiceCtx, db: &mut dyn Store) {
+    let Some(acc) = super::account(me, from, ctx) else { return };
+    let Some(name) = name else {
+        ctx.notice(me, from.uid, "Syntax: REGISTER <!group>");
+        return;
+    };
+    if !name.starts_with('!') || name.len() < 2 {
+        ctx.notice(me, from.uid, "A group name starts with \x02!\x02, e.g. \x02!staff\x02.");
+        return;
+    }
+    let acc = acc.to_string();
+    match db.group_register(name, &acc) {
+        Ok(()) => ctx.notice(me, from.uid, format!("Group \x02{name}\x02 registered — you're the founder.")),
+        Err(fedserv_api::ChanError::Exists) => ctx.notice(me, from.uid, format!("\x02{name}\x02 is already registered.")),
+        Err(_) => ctx.notice(me, from.uid, "Sorry, that didn't work. Please try again in a moment."),
+    }
+}