feat(app): shared offline-mode engine with connectivity actor and grace gate

New qbz_app::offline_mode (frontend-agnostic, ADR-006): three-state mode
machine (Online / RealOffline session-scoped / InducedOffline persisted),
Tauri-compatible offline_settings.db store with the issue #279
stream_first_track snapshot, and a layered connectivity actor (default-route
signal on Linux, audio liveness window, IP-literal + vendor-diverse strict
probes, asymmetric hysteresis with confirmation burst, captive-portal and
suspend/resume handling).

qbz-qobuz gains the process-wide offline_gate and ApiError::OfflineMode (the
single Qobuz choke point; HTTP funnel wiring lands next).
SubscriptionStateStore gains offline_playback_allowed: binary 30-day-grace
playback gate for the offline cache, never a preview-degradation path.

cc

crates/qbz-qobuz/src/error.rs

@@ -31,6 +31,9 @@ pub enum ApiError {
     #[error("Track {0} is no longer available on Qobuz")]
     TrackUnavailable(u64),
 
+    #[error("Offline mode is active - Qobuz services are disabled")]
+    OfflineMode,
+
     #[error("Network error: {0}")]
     NetworkError(#[from] reqwest::Error),
 

crates/qbz-qobuz/src/lib.rs

@@ -37,6 +37,7 @@ pub mod cmaf;
 pub mod endpoints;
 pub mod error;
 pub mod link_resolver;
+pub mod offline_gate;
 pub mod performers;
 pub mod retry;
 

crates/qbz-qobuz/src/offline_gate.rs

@@ -0,0 +1,47 @@
+//! Process-wide offline gate — the single Qobuz choke point.
+//!
+//! Offline MODE (induced from Settings or detected loss of connectivity)
+//! means ZERO Qobuz services: while the gate is closed, every HTTP request
+//! the client would issue fails fast with [`crate::error::ApiError::OfflineMode`]
+//! instead of timing out against the network. The owner of the flag is the
+//! shared offline-mode engine in `qbz-app`; frontends never set it directly.
+//!
+//! The flag is process-global on purpose: there is exactly one Qobuz client
+//! per process, the gate must hold across re-logins, and a per-instance flag
+//! would silently reset when the client is rebuilt.
+
+use std::sync::atomic::{AtomicBool, Ordering};
+
+static OFFLINE: AtomicBool = AtomicBool::new(false);
+
+/// Close (true) or open (false) the gate. Called by the offline-mode engine
+/// on every mode transition.
+pub fn set_offline(offline: bool) {
+    let was = OFFLINE.swap(offline, Ordering::Relaxed);
+    if was != offline {
+        log::info!(
+            "[OfflineGate] Qobuz API gate {}",
+            if offline { "CLOSED (offline mode)" } else { "OPEN" }
+        );
+    }
+}
+
+/// Whether the gate is currently closed (offline mode active).
+pub fn is_offline() -> bool {
+    OFFLINE.load(Ordering::Relaxed)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn gate_toggles() {
+        set_offline(false);
+        assert!(!is_offline());
+        set_offline(true);
+        assert!(is_offline());
+        set_offline(false);
+        assert!(!is_offline());
+    }
+}