feat(cli): références d'attachments [file n]/[image n] dans le transcript + zone chat redimensionnable — #188 (QA verte)

- Attachments: projection canonique ChatPromptAttachmentDto (référence [file n]/[image n] par compteur de type, ordre d'import) insérée dans le prompt modèle et transportée par ReplyChunk::UserPrompt (attachmentRefs) ; rendu compact côté CustomAgentChatView.
- Resize: la zone chat (LeafView/LayoutGrid) est redimensionnable, taille actuelle conservée par défaut, préférence locale persistée.
- Tests: commands (projection des refs), dto_chat, chat_bridge, CustomAgentChatView, LayoutGrid.chat.

crates/backend/src/dto.rs

@@ -3206,6 +3206,22 @@ pub struct ChatAttachmentDto {
     pub created_at: u64,
 }
 
+/// Canonical attachment projection attached to a streamed user prompt.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct ChatPromptAttachmentDto {
+    /// Stable textual reference inserted in the model prompt, e.g. `[file 1]`.
+    pub reference: String,
+    /// Original display filename.
+    pub filename: String,
+    /// MIME type.
+    pub mime: String,
+    /// Size in bytes.
+    pub size_bytes: u64,
+    /// Absolute managed path readable by the target agent sandbox.
+    pub readable_path: String,
+}
+
 impl From<domain::ChatAttachment> for ChatAttachmentDto {
     fn from(value: domain::ChatAttachment) -> Self {
         Self {
@@ -3359,6 +3375,9 @@ pub enum ReplyChunk {
         /// Display labels for attachments submitted with the prompt.
         #[serde(default, skip_serializing_if = "Vec::is_empty")]
         attachments: Vec<String>,
+        /// Canonical attachment references for this prompt, in backend import order.
+        #[serde(default, skip_serializing_if = "Vec::is_empty")]
+        attachment_refs: Vec<ChatPromptAttachmentDto>,
     },
     /// An assistant text fragment (incremental chat rendering).
     #[serde(rename_all = "camelCase")]