refactor: unify user mode processing into shared service layer

Both the HTTP API and IRC wire protocol handlers now call
service.ApplyUserMode/service.QueryUserMode for all user
mode operations. The service layer iterates mode strings
character by character (the correct IRC approach), ensuring
identical behavior regardless of transport.

Removed duplicate mode logic from internal/handlers/utility.go
(buildUserModeString, applyUserModeChange, applyModeChar) and
internal/ircserver/commands.go (buildUmodeString, inline iteration).

Added service-level tests for QueryUserMode, ApplyUserMode
(single-char, multi-char, invalid input, de-oper, +o rejection).

internal/ircserver/commands.go

@@ -730,17 +730,23 @@ func (c *Conn) handleUserMode(
 
 	// Mode query (no mode string).
 	if len(msg.Params) < 2 { //nolint:mnd
-		c.sendNumeric(
-			irc.RplUmodeIs,
-			c.buildUmodeString(ctx),
-		)
+		modes := c.svc.QueryUserMode(ctx, c.sessionID)
+		c.sendNumeric(irc.RplUmodeIs, modes)
 
 		return
 	}
 
-	modeStr := msg.Params[1]
+	newModes, err := c.svc.ApplyUserMode(
+		ctx, c.sessionID, msg.Params[1],
+	)
+	if err != nil {
+		var ircErr *service.IRCError
+		if errors.As(err, &ircErr) {
+			c.sendNumeric(ircErr.Code, ircErr.Message)
+
+			return
+		}
 
-	if len(modeStr) < 2 { //nolint:mnd
 		c.sendNumeric(
 			irc.ErrUmodeUnknownFlag,
 			"Unknown MODE flag",
@@ -749,64 +755,7 @@ func (c *Conn) handleUserMode(
 		return
 	}
 
-	adding := modeStr[0] == '+'
-
-	for _, ch := range modeStr[1:] {
-		switch ch {
-		case 'w':
-			_ = c.database.SetSessionWallops(
-				ctx, c.sessionID, adding,
-			)
-		case 'o':
-			if adding {
-				c.sendNumeric(
-					irc.ErrUmodeUnknownFlag,
-					"Unknown MODE flag",
-				)
-
-				return
-			}
-
-			_ = c.database.SetSessionOper(
-				ctx, c.sessionID, false,
-			)
-		default:
-			c.sendNumeric(
-				irc.ErrUmodeUnknownFlag,
-				"Unknown MODE flag",
-			)
-
-			return
-		}
-	}
-
-	c.sendNumeric(
-		irc.RplUmodeIs,
-		c.buildUmodeString(ctx),
-	)
-}
-
-// buildUmodeString returns the current user mode string.
-func (c *Conn) buildUmodeString(
-	ctx context.Context,
-) string {
-	modes := "+"
-
-	isOper, err := c.database.IsSessionOper(
-		ctx, c.sessionID,
-	)
-	if err == nil && isOper {
-		modes += "o"
-	}
-
-	isWallops, err := c.database.IsSessionWallops(
-		ctx, c.sessionID,
-	)
-	if err == nil && isWallops {
-		modes += "w"
-	}
-
-	return modes
+	c.sendNumeric(irc.RplUmodeIs, newModes)
 }
 
 // handleNames replies with channel member list.