Keep secrets in Credential Manager on Windows

The Secret Service store was keyring::Entry all the way down, and
keyring 4's v1 feature set — the one the workspace already asks for —
includes the Windows Credential Manager backend. So the Windows store
is the same implementation with its cfg widened, and the crate as a
target dependency. The one behavioural difference is that the
availability probe always succeeds there, which is correct: Credential
Manager is always present, so FR-NC-2's degraded mode does not arise.
Until now a Windows build compiled, started, and failed at sign-in
with the placeholder store's "no secret store is implemented".

platform/dr-plat/Cargo.toml

@@ -24,6 +24,11 @@ x11rb.workspace = true
 wayland-client.workspace = true
 wayland-protocols.workspace = true
 
+# The same crate on Windows: its `v1` features include the Credential Manager
+# backend, and `secrets.rs` is one implementation over both.
+[target.'cfg(windows)'.dependencies]
+keyring.workspace = true
+
 [target.'cfg(target_os = "android")'.dependencies]
 android-native-keyring-store.workspace = true
 keyring-core.workspace = true