feat(layout): supporte la création de layouts plugins (customPluginLayout)

Étend le flux de création de layout backend (DTO, usecases, store) et le
frontend (sélecteur, adaptateurs, LayoutTabs/LayoutGrid) pour permettre
d'ouvrir un layout déclaré par un plugin installé (ex. Android Health),
sans passer par le message bloquant "extension backend pas encore livrée".

Ticket #141 — QA vert.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

crates/backend/src/dto.rs

@@ -1118,9 +1118,9 @@ pub fn parse_session_id(raw: &str) -> Result<SessionId, ErrorDto> {
 
 use application::{
     CreateLayoutOutput, DeleteLayoutOutput, LayoutInfo, LayoutOperation, ListLayoutsOutput,
-    LoadLayoutOutput, MutateLayoutOutput, SetActiveLayoutOutput,
+    LoadLayoutOutput, MutateLayoutOutput, PluginLayoutOrigin, SetActiveLayoutOutput,
 };
-use domain::{AgentId, Direction, LayoutId, LayoutTree, NodeId};
+use domain::{AgentId, Direction, LayoutId, LayoutTree, NodeId, PluginId, PluginLayoutType};
 
 /// Response DTO carrying a layout tree.
 ///
@@ -1214,6 +1214,15 @@ pub enum LayoutOperationDto {
         #[serde(default)]
         conversation_id: Option<String>,
     },
+    /// Persist opaque plugin layout state.
+    #[serde(rename_all = "camelCase")]
+    SetPluginLayoutState {
+        /// Custom plugin layout node.
+        target: String,
+        /// Opaque plugin-owned state.
+        #[serde(default)]
+        state: serde_json::Value,
+    },
 }
 
 impl LayoutOperationDto {
@@ -1264,6 +1273,10 @@ impl LayoutOperationDto {
                 target: parse_node_id(&target)?,
                 conversation_id,
             },
+            Self::SetPluginLayoutState { target, state } => LayoutOperation::SetPluginLayoutState {
+                target: parse_node_id(&target)?,
+                state,
+            },
         })
     }
 }
@@ -1306,7 +1319,7 @@ pub struct LayoutInfoDto {
     pub id: String,
     /// Display name.
     pub name: String,
-    /// Layout kind: `"terminal"` or `"gitGraph"`.
+    /// Layout kind: `"terminal"`, `"gitGraph"` or `"plugin"`.
     pub kind: String,
 }
 
@@ -1315,6 +1328,7 @@ impl From<LayoutInfo> for LayoutInfoDto {
         let kind = match info.kind {
             LayoutKind::Terminal => "terminal",
             LayoutKind::GitGraph => "gitGraph",
+            LayoutKind::Plugin { .. } => "plugin",
         }
         .to_owned();
         Self {
@@ -1384,9 +1398,15 @@ pub struct CreateLayoutRequestDto {
     pub project_id: String,
     /// Display name for the new layout.
     pub name: String,
-    /// Optional layout kind: `"terminal"` (default) or `"gitGraph"`.
+    /// Optional layout kind: `"terminal"` (default), `"gitGraph"` or `"plugin"`.
     #[serde(default)]
     pub kind: Option<String>,
+    /// Plugin origin for `"plugin"` layouts.
+    #[serde(default)]
+    pub plugin_origin: Option<PluginLayoutOriginDto>,
+    /// Opaque initial state for `"plugin"` layouts.
+    #[serde(default)]
+    pub state: serde_json::Value,
 }
 
 impl CreateLayoutRequestDto {
@@ -1398,6 +1418,16 @@ impl CreateLayoutRequestDto {
         match self.kind.as_deref() {
             None | Some("terminal") => Ok(LayoutKind::Terminal),
             Some("gitGraph") => Ok(LayoutKind::GitGraph),
+            Some("plugin") | Some("customPluginLayout") => {
+                let origin = self.plugin_origin.clone().ok_or_else(|| ErrorDto {
+                    code: "INVALID".to_owned(),
+                    message: "pluginOrigin is required for plugin layouts".to_owned(),
+                })?;
+                Ok(LayoutKind::Plugin {
+                    plugin_origin: origin.try_into()?,
+                    state: self.state.clone(),
+                })
+            }
             Some(other) => Err(ErrorDto {
                 code: "INVALID".to_owned(),
                 message: format!("unknown layout kind: {other}"),
@@ -1406,6 +1436,35 @@ impl CreateLayoutRequestDto {
     }
 }
 
+/// Stable provider identity for a plugin layout creation request.
+#[derive(Debug, Clone, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct PluginLayoutOriginDto {
+    /// Provider plugin id.
+    pub plugin_id: String,
+    /// Layout type declared by the provider.
+    pub layout_type: String,
+}
+
+impl TryFrom<PluginLayoutOriginDto> for PluginLayoutOrigin {
+    type Error = ErrorDto;
+
+    fn try_from(value: PluginLayoutOriginDto) -> Result<Self, Self::Error> {
+        let plugin_id = PluginId::new(value.plugin_id).map_err(|e| ErrorDto {
+            code: "INVALID".to_owned(),
+            message: e.to_string(),
+        })?;
+        let layout_type = PluginLayoutType::new(value.layout_type).map_err(|e| ErrorDto {
+            code: "INVALID".to_owned(),
+            message: e.to_string(),
+        })?;
+        Ok(Self {
+            plugin_id,
+            layout_type,
+        })
+    }
+}
+
 /// Request DTO for `rename_layout`.
 #[derive(Debug, Clone, Deserialize)]
 #[serde(rename_all = "camelCase")]