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.

client-desktop/Cargo.toml

@@ -4,9 +4,12 @@ version.workspace = true
 edition.workspace = true
 description = "Desktop dictation client for the doctate medical dictation system"
 
+[[bin]]
+name = "doctate"
+path = "src/main.rs"
+
 [dependencies]
 doctate-common = { path = "../doctate-common" }
-doctate-client-core = { path = "../doctate-client-core" }
 eframe = "0.28"
 reqwest = { workspace = true }
 serde = { workspace = true }
@@ -29,3 +32,4 @@ libc = "0.2"
 tempfile = "3"
 tokio = { workspace = true, features = ["test-util"] }
 wiremock = "0.6"
+filetime = "0.2"