dsh-session-search-toggle: Add session content search to DSH web sidebar

dsh-session-search-toggle is an MIT-licensed plugin maintained by drscrewdriver, designed to enhance the session retrieval capabilities of the DeepSeek Harness (DSH) web sidebar. Leveraging the existing sessionQuery service without modifying the official source code, the plugin provides dual-mode search for titles and content. The title mode supports instant filtering by title or directory; the content mode reuses the FTS5 full-text index to search message bodies and supports filtering by "All, User, Reply, Tool". Search results are aggregated by session, displaying the strongest matching snippets that can be clicked to locate the context. The plugin automatically detects the full-text index status and provides configuration guidance if it is not enabled (requires configuring session-query-sqli).