feat(downloads): WiFi-only network-type-aware download gating

Add a metered/cellular network detector so downloads honour a "WiFi
only" preference. Android reports network type via NetworkTypeMonitor;
Rust exposes it through download/network.rs and holds the queue pump when
on a metered connection, emitting a queue-wide waitingForNetwork event.
The frontend surfaces this via the networkType service and a
waitingForNetwork store flag.

TRACES: UR-053 | DR-074

src-tauri/src/download/network.rs

@@ -0,0 +1,196 @@
+//! Network transport classification for the WiFi-only download gate.
+//!
+//! This answers "what kind of connection are we on?", which is orthogonal to
+//! the `ConnectivityMonitor`'s "is the server reachable?". The download queue
+//! pump consults this before starting pending rows when the user has enabled
+//! WiFi-only downloads.
+//!
+//! On Android the real transport is read from `NetworkCapabilities` in
+//! `NetworkTypeMonitor.kt` and pushed in from the frontend. On desktop there is
+//! no metered-connection concept worth enforcing, so we report `Ethernet`,
+//! which is always acceptable — gating desktop downloads would be a regression.
+
+use serde::{Deserialize, Serialize};
+use std::sync::Arc;
+use tokio::sync::RwLock;
+
+/// Kind of network transport currently active.
+///
+/// Mirrors the string constants in `NetworkTypeMonitor.kt`; the two must stay
+/// in sync (the serde rename below is what the frontend sends).
+///
+/// TRACES: UR-053 | DR-074
+#[derive(specta::Type, Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "lowercase")]
+pub enum NetworkType {
+    /// No active network.
+    None,
+    /// WiFi (may still be metered — check `unmetered`).
+    Wifi,
+    /// Wired ethernet, typical on Android TV and desktop.
+    Ethernet,
+    /// Mobile data — never acceptable when wifi-only is enabled.
+    Cellular,
+    /// Some other transport (VPN over unknown carrier, Bluetooth tethering, …).
+    Other,
+    /// Could not determine the transport.
+    Unknown,
+}
+
+/// Current network transport plus whether it is metered.
+///
+/// TRACES: UR-053 | DR-074
+#[derive(specta::Type, Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct NetworkState {
+    pub network_type: NetworkType,
+    /// Whether the active network is unmetered (Android `NET_CAPABILITY_NOT_METERED`).
+    pub unmetered: bool,
+}
+
+impl Default for NetworkState {
+    fn default() -> Self {
+        // Desktop default: wired and unmetered, so the gate never blocks there.
+        // Android overwrites this as soon as the frontend reports the real state.
+        Self {
+            network_type: NetworkType::Ethernet,
+            unmetered: true,
+        }
+    }
+}
+
+impl NetworkState {
+    /// Whether downloads may run right now given the wifi-only preference.
+    ///
+    /// Ethernet counts as acceptable — it is unmetered in practice and is what
+    /// Android TV devices use. Cellular never does. `None`/`Unknown` fail
+    /// closed: if we cannot tell what we are on, we do not spend the user's
+    /// mobile data to find out.
+    ///
+    /// TRACES: UR-053 | DR-074
+    pub fn allows_download(&self, wifi_only: bool) -> bool {
+        if !wifi_only {
+            return true;
+        }
+        match self.network_type {
+            NetworkType::Cellular | NetworkType::None | NetworkType::Unknown => false,
+            // Require unmetered so metered WiFi hotspots (backed by the very
+            // cellular data this setting protects) are excluded too.
+            NetworkType::Wifi | NetworkType::Ethernet | NetworkType::Other => self.unmetered,
+        }
+    }
+}
+
+/// Shared, mutable view of the current network transport.
+///
+/// Cheap to clone; the frontend updates it via `set_network_state` whenever
+/// Android reports a network change.
+#[derive(Clone, Default)]
+pub struct NetworkStateHandle {
+    state: Arc<RwLock<NetworkState>>,
+}
+
+impl NetworkStateHandle {
+    pub fn new() -> Self {
+        Self {
+            state: Arc::new(RwLock::new(NetworkState::default())),
+        }
+    }
+
+    pub async fn get(&self) -> NetworkState {
+        *self.state.read().await
+    }
+
+    pub async fn set(&self, new_state: NetworkState) {
+        *self.state.write().await = new_state;
+    }
+
+    /// Whether downloads may run right now given the wifi-only preference.
+    pub async fn allows_download(&self, wifi_only: bool) -> bool {
+        self.state.read().await.allows_download(wifi_only)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn state(network_type: NetworkType, unmetered: bool) -> NetworkState {
+        NetworkState {
+            network_type,
+            unmetered,
+        }
+    }
+
+    #[test]
+    fn wifi_only_off_allows_every_transport() {
+        for t in [
+            NetworkType::None,
+            NetworkType::Wifi,
+            NetworkType::Ethernet,
+            NetworkType::Cellular,
+            NetworkType::Other,
+            NetworkType::Unknown,
+        ] {
+            assert!(
+                state(t, false).allows_download(false),
+                "{t:?} should be allowed when wifi_only is off"
+            );
+        }
+    }
+
+    #[test]
+    fn cellular_is_blocked_when_wifi_only() {
+        // Even if somehow flagged unmetered, cellular is never acceptable.
+        assert!(!state(NetworkType::Cellular, true).allows_download(true));
+        assert!(!state(NetworkType::Cellular, false).allows_download(true));
+    }
+
+    #[test]
+    fn unmetered_wifi_and_ethernet_are_allowed() {
+        assert!(state(NetworkType::Wifi, true).allows_download(true));
+        assert!(state(NetworkType::Ethernet, true).allows_download(true));
+    }
+
+    #[test]
+    fn metered_wifi_is_blocked() {
+        // A phone hotspot reports as WiFi but is metered — blocking it is the
+        // whole point of checking NOT_METERED rather than the transport alone.
+        assert!(!state(NetworkType::Wifi, false).allows_download(true));
+    }
+
+    #[test]
+    fn unknown_and_none_fail_closed() {
+        assert!(!state(NetworkType::Unknown, true).allows_download(true));
+        assert!(!state(NetworkType::None, true).allows_download(true));
+    }
+
+    #[test]
+    fn desktop_default_is_never_gated() {
+        assert!(NetworkState::default().allows_download(true));
+    }
+
+    #[tokio::test]
+    async fn handle_roundtrips_state() {
+        let handle = NetworkStateHandle::new();
+        assert!(handle.allows_download(true).await);
+
+        handle.set(state(NetworkType::Cellular, false)).await;
+        assert!(!handle.allows_download(true).await);
+        assert!(handle.allows_download(false).await);
+        assert_eq!(handle.get().await.network_type, NetworkType::Cellular);
+    }
+
+    #[test]
+    fn network_type_serializes_lowercase() {
+        // Must match the string constants in NetworkTypeMonitor.kt.
+        assert_eq!(
+            serde_json::to_string(&NetworkType::Wifi).unwrap(),
+            "\"wifi\""
+        );
+        assert_eq!(
+            serde_json::to_string(&NetworkType::Cellular).unwrap(),
+            "\"cellular\""
+        );
+    }
+}