fix: update HTTP client to flare for Batch 4 sources

Based on Kotlin reference verification:
- madtheme, mangabox, mangacatalog, mangahub → flare

Also updated phase4-standalone.md with project requirement for
checking HTTP client type when porting sources from Kotlin

sources/base/madtheme/madtheme.go

@@ -14,7 +14,7 @@ import (
 
 	"github.com/PuerkitoBio/goquery"
 
-	"goyomi/internal/httpclient"
+	"goyomi/internal/httpclient/flare"
 	"goyomi/internal/source"
 	"goyomi/sources/base/util"
 )
@@ -27,12 +27,12 @@ type Config struct {
 
 type Source struct {
 	cfg    Config
-	client *httpclient.Client
+	client *flare.Client
 	id     int64
 }
 
 func New(cfg Config) *Source {
-	c := httpclient.NewClient(httpclient.WithRateLimit(1, 2))
+	c := flare.NewClient(flare.WithRateLimit(1, 2))
 	return &Source{cfg: cfg, client: c, id: source.GenerateSourceID(cfg.Name, cfg.Lang)}
 }
 
@@ -42,12 +42,7 @@ func (s *Source) Lang() string        { return s.cfg.Lang }
 func (s *Source) SupportsLatest() bool { return true }
 
 func (s *Source) get(ctx context.Context, rawURL string) (*goquery.Document, error) {
-	req, err := http.NewRequestWithContext(ctx, http.MethodGet, rawURL, nil)
-	if err != nil {
-		return nil, err
-	}
-	req.Header.Set("Referer", s.cfg.BaseURL+"/")
-	resp, err := s.client.Do(req)
+	resp, err := s.client.Get(ctx, rawURL)
 	if err != nil {
 		return nil, err
 	}