feat(backend,application,frontend,domain,web-server,app-tauri): bouton Redémarrer l'agent dans la barre supérieure des cellules — #317 (QA verte)

La cellule expose un bouton Redémarrer l'agent valable pour les trois
modes de présentation (headless, TUI native, CLI custom), sans jamais
prendre la présentation pour vérité runtime.

- Application: use case RestartCellAgentRuntime validant le binding
  exact cellule/runtime via les snapshots live (RestartRuntimeNotFound,
  ConflictAgentVisibleElsewhere, AgentNotBoundToCell) puis déléguant au
  scheduler self-restart existant (schedule_cell); CellAgentPresentation
  purement informationnelle.
- Backend/app-tauri/web-server: commande restart_cell_agent sur contrat
  DTO identique Tauri/HTTP, stop borné par timeout typé
  (RestartStopTimeout), relance préservant routage TUI natif, géométrie
  de cellule (node/rows/cols) et conversation; évenements domaine
  enrichis.
- Frontend: bouton dans la barre de cellule avec confirmation,
  overlay d'état Redémarrage…/échec (libellés par code d'erreur, Fermer/
  Réessayer), gardes d'identité de cellule et de restarts externes,
  gateway/mock/http adapters et ports alignés.
- Tests: dto_agents, workstate_actions, contrats web-server,
  LayoutGrid.cellControlsLayering, adapters agent/mock/http.

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

crates/application/src/error.rs

@@ -73,6 +73,36 @@ pub enum AppError {
         node_id: NodeId,
     },
 
+    /// No live runtime can be restarted for the requested agent.
+    #[error("runtime not found for agent {agent_id}")]
+    RestartRuntimeNotFound {
+        /// Agent whose runtime was requested.
+        agent_id: AgentId,
+    },
+
+    /// The requested cell does not host the requested live agent runtime.
+    #[error("agent {agent_id} is not bound to cell {node_id}")]
+    AgentNotBoundToCell {
+        /// Agent supplied by the caller.
+        agent_id: AgentId,
+        /// Cell supplied by the caller.
+        node_id: NodeId,
+    },
+
+    /// More than one live runtime/cell currently represents the same agent.
+    #[error("agent {agent_id} is visible in more than one live cell")]
+    ConflictAgentVisibleElsewhere {
+        /// Ambiguous agent.
+        agent_id: AgentId,
+    },
+
+    /// Runtime teardown did not reach its terminal boundary in time.
+    #[error("timed out while stopping agent {agent_id} for restart")]
+    RestartStopTimeout {
+        /// Agent whose old runtime could not be stopped conclusively.
+        agent_id: AgentId,
+    },
+
     /// The delegation target finished its structured/headless turn without producing
     /// a [`ReplyEvent::Final`](domain::ports::ReplyEvent::Final), so the rendezvous
     /// could not capture an inline answer. Surfaced promptly instead of blocking the
@@ -120,6 +150,10 @@ impl AppError {
             Self::Git(_) => "GIT",
             Self::Remote(_) => "REMOTE",
             Self::AgentAlreadyRunning { .. } => "AGENT_ALREADY_RUNNING",
+            Self::RestartRuntimeNotFound { .. } => "RUNTIME_NOT_FOUND",
+            Self::AgentNotBoundToCell { .. } => "AGENT_NOT_BOUND_TO_CELL",
+            Self::ConflictAgentVisibleElsewhere { .. } => "CONFLICT_AGENT_VISIBLE_ELSEWHERE",
+            Self::RestartStopTimeout { .. } => "STOP_TIMEOUT",
             Self::TargetReturnedNoReply(_) => "TARGET_RETURNED_NO_REPLY",
             Self::TargetCeilingActive(_) => "RENDEZVOUS_CEILING_ACTIVE",
             Self::Internal(_) => "INTERNAL",