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-app/Cargo.toml

@@ -10,6 +10,8 @@ qbz-models = { path = "../qbz-models" }
 qbz-audio = { path = "../qbz-audio" }
 qbz-player = { path = "../qbz-player" }
 qbz-core = { path = "../qbz-core" }
+qbz-qobuz = { path = "../qbz-qobuz" }
+reqwest = { workspace = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
 tokio = { workspace = true }