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

docs/phase4-standalone.md

@@ -585,6 +585,13 @@ Detailed implementation notes for complex sources are in the **Notes** section a
 
 ## Notes — Complex Sources
 
+### Project Requirement: HTTP Client Selection
+When porting sources from Kotlin to Go, ALWAYS check the Kotlin reference to determine whether the source needs:
+- **Normal HTTP client** (`goyomi/internal/httpclient`) - for REST APIs returning JSON
+- **FlareSolverr client** (`goyomi/internal/httpclient/flare`) - for JavaScript-rendered pages (uses `network.cloudflareClient` in Kotlin)
+
+Check in Kotlin: `override val client = network.cloudflareClient` or `network.cloudflareClient.newBuilder()` → needs FlareSolverr
+
 ### `all/mangadex` ⚠️
 - Rate limit: 5 req/s (`golang.org/x/time/rate`)
 - `GetPopularManga` — `GET /manga?order[rating]=desc&includes[]=cover_art&limit=20&offset={(n-1)*20}`