refactor: dissolve doctate-client-core into doctate-desktop

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.

Cargo.lock

@@ -1145,25 +1145,6 @@ dependencies = [
  "libloading 0.8.9",
 ]
 
-[[package]]
-name = "doctate-client-core"
-version = "0.1.0"
-dependencies = [
- "doctate-common",
- "filetime",
- "reqwest",
- "serde",
- "serde_json",
- "tempfile",
- "thiserror 1.0.69",
- "time",
- "tokio",
- "toml",
- "tracing",
- "uuid",
- "wiremock",
-]
-
 [[package]]
 name = "doctate-common"
 version = "0.1.0"
@@ -1179,9 +1160,9 @@ name = "doctate-desktop"
 version = "0.1.0"
 dependencies = [
  "directories",
- "doctate-client-core",
  "doctate-common",
  "eframe",
+ "filetime",
  "libc",
  "reqwest",
  "serde",