refactor(audit): batch 7 — RPC/settings/lite/effects mechanical dedups

Six behavior-preserving consolidations from the audit:

- RPCClient::call() overloads share a private performCall() (payload dump +
  curl_easy_perform + http code) and a static parseRpcResult() (error->RpcError
  extraction). The timeout overload restores the prior timeout in both the
  success and catch paths, exactly as before.
- The six UnifiedCallback methods delegate to one splitUnified() that builds
  the (Callback, ErrorCallback) pair once (null-check preserved).
- One liteTrimCopy() in lite_connection_service replaces 5 file-local
  trim-copy helpers across the lite slice.
- lite_wallet_controller's inline height JSON parse now routes through the
  tested parseLiteHeightResponse().
- theme_effects.cpp: unpackRGB()/scaledAlpha() replace 14 RGB-unpack + 5
  alpha-scale copy-paste blocks.
- settings.cpp load() uses loadScalar()/loadClamped() helpers for ~44 scalar
  fields. Besides removing boilerplate this HARDENS loading: ~44 fields that
  previously only checked contains() now also verify the JSON type, so a
  malformed value in settings.json falls back to the default instead of
  throwing/misreading. Custom cases (enum parses, legacy migrations, arrays)
  stay inline; save() is unchanged.

Full-node + Lite build clean; ctest 1/1; hygiene clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/rpc/rpc_client.h

@@ -245,6 +245,17 @@ private:
     json makePayload(const std::string& method, const json& params = json::array());
     void doRPC(const std::string& method, const json& params, Callback cb, ErrorCallback err);
 
+    // Shared blocking-call core: dumps the payload, performs the request (throwing on a
+    // curl transport error) and returns the response body plus HTTP status. Caller must
+    // hold curl_mutex_ and have verified impl_->curl.
+    std::string performCall(const std::string& method, const json& params, long& httpCodeOut);
+    // Centralizes the HTTP-code check and JSON error->RpcError extraction, returning
+    // response["result"] on success.
+    static json parseRpcResult(long httpCode, const std::string& body);
+
+    // Splits a UnifiedCallback into the (Callback, ErrorCallback) pair used by doRPC.
+    static std::pair<Callback, ErrorCallback> splitUnified(UnifiedCallback cb);
+
     std::string host_;
     std::string port_;
     std::string auth_;  // Base64 encoded "user:password"