Move the 9 client-core modules (case_store, case_update, config,
footer_status, pending_cleanup, server_sync, snapshot_cache,
startup, upload) into client-desktop/src/. Resolve the config.rs
name collision by keeping the TOML schema in config.rs (was core)
and renaming the platform-path wrapper to config_path.rs.
Introduce client-desktop/src/lib.rs so the modules are reachable
under the doctate-desktop:: crate path -- needed for Wear-OS doc
anchors that mirror Rust symbols. Bin target stays as `doctate`
via [[bin]] name in Cargo.toml.
This is the first of four stages preparing the server-vs-clients
build-world split. Workspace still has 3 members; directory layout
unchanged. All 508 workspace tests pass.
