Add TOML dependency and new config/startup modules

The TOML dependency is added to `Cargo.lock` to support configuration
file parsing.
New modules `config` and `startup` are introduced in
`doctate-client-core`:
- `config`: Handles client configuration loading and saving, including
  defaults for polling intervals and window hours.
- `startup`: Contains routines for initial cleanup tasks, such as
  removing stale markers and orphan audio files based on defined
  retention periods.

doctate-client-core/Cargo.toml

@@ -2,7 +2,7 @@
 name = "doctate-client-core"
 version.workspace = true
 edition.workspace = true
-description = "Shared client logic for the doctate medical dictation system: case store, oneliner poller, snapshot cache"
+description = "Shared client logic for the doctate medical dictation system: case store, server sync worker, config, snapshot cache"
 
 [dependencies]
 doctate-common = { path = "../doctate-common" }
@@ -10,6 +10,7 @@ tokio = { workspace = true }
 reqwest = { workspace = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
+toml = { workspace = true }
 uuid = { workspace = true }
 time = { workspace = true }
 tracing = { workspace = true }