feat: Add LSP plugin unit tests and frontend common initialization library

- Introduced `dstalk_lsp_plugin_test` for testing LSP plugin functionalities including `lsp_trim`, `lsp_frame_message`, and `lsp_parse_content_length`.
- Created `dstalk_frontend_common` static library to encapsulate shared initialization logic for frontend components (CLI, GUI, Web).
- Implemented configuration file discovery and service querying in `dstalk_frontend_init`.
- Added internal headers for LSP and Anthropic plugins to facilitate unit testing.
- Established a mailroom system for asynchronous message passing between stateless agents, enhancing coordination and context management.

dstalk_gui/CMakeLists.txt

@@ -16,5 +16,6 @@ set_target_properties(dstalk_gui PROPERTIES
 target_link_libraries(dstalk_gui
     PRIVATE
         dstalk
+        dstalk_frontend_common
         SDL3::SDL3
 )