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

platform/dr-plat/src/secrets.rs

@@ -100,14 +100,22 @@ pub trait SecretStore: Send + Sync {
 ///
 /// Used by the two stores that have somewhere to file them; the placeholder
 /// below has nothing to name, and a Windows build otherwise warns here.
-#[cfg(any(all(unix, not(target_os = "android")), target_os = "android"))]
+#[cfg(any(all(unix, not(target_os = "android")), windows, target_os = "android"))]
 const SERVICE: &str = "DarkRoom";
 
-/// Secret Service implementation (GNOME Keyring, KWallet via ksecretd).
+/// The `keyring`-backed store: Secret Service on Linux (GNOME Keyring,
-#[cfg(all(unix, not(target_os = "android")))]
+/// KWallet via ksecretd), Credential Manager on Windows.
+///
+/// One implementation for both because `keyring::Entry` is the same API over
+/// either, and its `v1` feature set already includes the Windows backend. The
+/// difference is in what "unavailable" means: a Linux desktop may have no
+/// secrets daemon, which FR-NC-2 treats as a stated degraded mode, while
+/// Credential Manager is always present — so on Windows `is_available` is a
+/// probe that always succeeds, and that is correct rather than optimistic.
+#[cfg(any(all(unix, not(target_os = "android")), windows))]
 pub struct PlatformSecretStore;
 
-#[cfg(all(unix, not(target_os = "android")))]
+#[cfg(any(all(unix, not(target_os = "android")), windows))]
 impl PlatformSecretStore {
     pub fn new() -> Self {
         Self
@@ -118,14 +126,14 @@ impl PlatformSecretStore {
     }
 }
 
-#[cfg(all(unix, not(target_os = "android")))]
+#[cfg(any(all(unix, not(target_os = "android")), windows))]
 impl Default for PlatformSecretStore {
     fn default() -> Self {
         Self::new()
     }
 }
 
-#[cfg(all(unix, not(target_os = "android")))]
+#[cfg(any(all(unix, not(target_os = "android")), windows))]
 impl SecretStore for PlatformSecretStore {
     fn store(&self, secret_ref: &SecretRef, secret: &str) -> Result<(), SecretError> {
         Self::entry(secret_ref)?
@@ -160,7 +168,7 @@ impl SecretStore for PlatformSecretStore {
     }
 }
 
-#[cfg(all(unix, not(target_os = "android")))]
+#[cfg(any(all(unix, not(target_os = "android")), windows))]
 fn map_err(e: keyring::Error) -> SecretError {
     match e {
         keyring::Error::NoEntry => SecretError::NotFound,
@@ -265,24 +273,24 @@ fn map_err(e: keyring_core::Error) -> SecretError {
 ///
 /// Failing loudly is deliberate: a silent no-op store would look like it
 /// worked and then lose the credential.
-#[cfg(not(any(all(unix, not(target_os = "android")), target_os = "android")))]
+#[cfg(not(any(all(unix, not(target_os = "android")), windows, target_os = "android")))]
 pub struct PlatformSecretStore;
 
-#[cfg(not(any(all(unix, not(target_os = "android")), target_os = "android")))]
+#[cfg(not(any(all(unix, not(target_os = "android")), windows, target_os = "android")))]
 impl PlatformSecretStore {
     pub fn new() -> Self {
         Self
     }
 }
 
-#[cfg(not(any(all(unix, not(target_os = "android")), target_os = "android")))]
+#[cfg(not(any(all(unix, not(target_os = "android")), windows, target_os = "android")))]
 impl Default for PlatformSecretStore {
     fn default() -> Self {
         Self::new()
     }
 }
 
-#[cfg(not(any(all(unix, not(target_os = "android")), target_os = "android")))]
+#[cfg(not(any(all(unix, not(target_os = "android")), windows, target_os = "android")))]
 impl SecretStore for PlatformSecretStore {
     fn store(&self, _r: &SecretRef, _s: &str) -> Result<(), SecretError> {
         Err(SecretError::Unavailable(