refactor(guild): introduce service layer for guild member operations

Extract business logic from handleMsgMhfOperateGuildMember into
GuildService.OperateMember, establishing the handler→service→repo
layering pattern. The handler is now ~20 lines of protocol glue
(type-assert, map action, call service, send ACK, notify).

GuildService owns authorization checks, repo coordination, mail
composition, and best-effort mail delivery. It accepts plain Go
types (no mhfpacket or Session imports), making it fully testable
with mock repos. Cross-channel notification stays in the handler
since it requires Session.

Adds 7 table-driven service-level tests covering accept/reject/kick,
authorization, repo errors, mail errors, and unknown actions.

server/channelserver/test_helpers_test.go

@@ -51,9 +51,16 @@ func createMockServer() *Server {
 	}
 	s.i18n = getLangStrings(s)
 	s.Registry = NewLocalChannelRegistry([]*Server{s})
+	// GuildService is wired lazily by tests that set repos then call ensureGuildService.
 	return s
 }
 
+// ensureGuildService wires the GuildService from the server's current repos.
+// Call this after setting guildRepo, mailRepo, and charRepo on the mock server.
+func ensureGuildService(s *Server) {
+	s.guildService = NewGuildService(s.guildRepo, s.mailRepo, s.charRepo, s.logger)
+}
+
 // createMockSession creates a minimal Session for testing.
 // Imported from v9.2.x-stable and adapted for main.
 func createMockSession(charID uint32, server *Server) *Session {