[rfprod]

• use std::shared_ptr for UserInterface, ProgressSpinner, and component ownership;
• store the UserInterface singleton as std::weak_ptr, add create() factory overloads;
• create UserInterface through create() and allocate components with std::make_shared;
• initialize OllamaClient and its members through constructor initializer lists;
• accept shared pointers in CurlUtils, EmbeddingUtils, RetrievingUtils, SystemUtils;
• replace the SIGINT handler's direct exit with a graceful interrupt flag;
• make resize and interrupt flags static volatile sig_atomic_t members;
• add a terminal_restored flag and guard terminal cleanup against repeated calls;
• prevent self-join in stop_render_thread;
• add static ui_running() helper and use it for UI state checks in all components;
• add a default exit code to end_session() and rename its parameter;
• replace exit() calls with std::exit(EXIT_SUCCESS) or std::exit(EXIT_FAILURE);
• return EXIT_SUCCESS from main and end UI sessions with EXIT_FAILURE on fatal errors;
• hold CURL easy handles and header lists in unique_ptr with custom deleters;
• add CurlUtils::add_header() to append headers to a curl_slist safely;
• use reset() on owned CURL pointers in place of manual cleanup calls;
• initialize WebContent::error_buffer with {} so it starts zeroed;
• remove DEBUG-only theme logging from init_ncurses();
• add doc comments for client, CurlUtils, EmbeddingUtils, and RetrievingUtils methods;
• document fetch_web_content_stream() and fix the @throws note in fetch_web_content();
• remove the is_running() note from catch_error() documentation;