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/drop.rs

@@ -0,0 +1,22 @@
+use fedserv_api::{Sender, ServiceCtx, Store};
+
+// DROP <!group>: delete a group. Founder only.
+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: DROP <!group>");
+        return;
+    };
+    let Some(g) = db.group(name) else {
+        ctx.notice(me, from.uid, format!("\x02{name}\x02 isn't registered."));
+        return;
+    };
+    if !g.founder.eq_ignore_ascii_case(acc) {
+        ctx.notice(me, from.uid, format!("Only \x02{name}\x02's founder can drop it."));
+        return;
+    }
+    match db.group_drop(name) {
+        Ok(()) => ctx.notice(me, from.uid, format!("Group \x02{name}\x02 has been dropped.")),
+        Err(_) => ctx.notice(me, from.uid, "Sorry, that didn't work. Please try again in a moment."),
+    }
+}