feat(windows): persistance et restauration des fenêtres au redémarrage (#40)

Persiste l'état des fenêtres (layout/position) et le restaure au
relancement de l'application. Découpage hexagonal :
- domaine : modèle et port d'état des fenêtres (layout, ports)
- application : use cases de persistance/restauration
- infrastructure : store window_state (adapter de persistance)
- présentation : câblage app-tauri (state, commands, lib)
Couvert par des tests ciblés domaine/application/infrastructure.

Depend de #39 (fermeture des fenêtres auxiliaires).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/domain/src/ports.rs

@@ -1417,6 +1417,26 @@ pub trait ProjectStore: Send + Sync {
     async fn load_workspace(&self) -> Result<crate::layout::Workspace, StoreError>;
 }
 
+/// Machine-local persistence of open OS window state.
+#[async_trait]
+pub trait WindowStateStore: Send + Sync {
+    /// Saves the latest open-window snapshot.
+    ///
+    /// # Errors
+    /// [`StoreError`] on persistence failure.
+    async fn save_window_state(
+        &self,
+        snapshot: &crate::layout::WindowStateSnapshot,
+    ) -> Result<(), StoreError>;
+
+    /// Loads the latest open-window snapshot.
+    ///
+    /// # Errors
+    /// [`StoreError`] on unexpected I/O or deserialisation failure. A missing
+    /// snapshot is represented as an empty default snapshot.
+    async fn load_window_state(&self) -> Result<crate::layout::WindowStateSnapshot, StoreError>;
+}
+
 /// CRUD for the configured [`AgentProfile`]s in the global IDE store
 /// (`profiles.json`, ARCHITECTURE §9.2). Profiles are the *data* that drives the
 /// single generic [`AgentRuntime`] adapter (Open/Closed).