feat(chat): #155 paste image depuis presse-papier dans le composer custom (QA verte)

- frontend: onPaste sur CustomAgentChatView, détection MIME image, chip/preview d'attachment, envoi via le contrat #154 (adapters/agent, ports)

- backend: import d'image par bytes dans le store attachments (commands, chat_attachments app+infra, dto, ports) + tests

crates/app-tauri/tests/dto_chat.rs

@@ -119,7 +119,26 @@ fn chat_attachment_input_dto_deserialises_camel_case() {
     }))
     .unwrap();
 
-    assert_eq!(dto.path, "/tmp/picked.png");
+    assert_eq!(dto.path.as_deref(), Some("/tmp/picked.png"));
+    assert_eq!(dto.filename, None);
+    assert_eq!(dto.content_base64, None);
+    assert_eq!(dto.mime.as_deref(), Some("image/png"));
+    assert_eq!(dto.source_kind, Some(ChatAttachmentSourceKind::Clipboard));
+}
+
+#[test]
+fn chat_attachment_input_dto_deserialises_clipboard_bytes_camel_case() {
+    let dto: ChatAttachmentInputDto = serde_json::from_value(json!({
+        "filename": "clipboard.png",
+        "contentBase64": "cG5nIGJ5dGVz",
+        "mime": "image/png",
+        "sourceKind": "clipboard"
+    }))
+    .unwrap();
+
+    assert_eq!(dto.path, None);
+    assert_eq!(dto.filename.as_deref(), Some("clipboard.png"));
+    assert_eq!(dto.content_base64.as_deref(), Some("cG5nIGJ5dGVz"));
     assert_eq!(dto.mime.as_deref(), Some("image/png"));
     assert_eq!(dto.source_kind, Some(ChatAttachmentSourceKind::Clipboard));
 }
@@ -154,6 +173,11 @@ fn import_chat_attachments_request_response_use_camel_case() {
             "path": "/tmp/picked.png",
             "mime": "image/png",
             "sourceKind": "dragDrop"
+        }, {
+            "filename": "clipboard.png",
+            "contentBase64": "cG5nIGJ5dGVz",
+            "mime": "image/png",
+            "sourceKind": "clipboard"
         }]
     }))
     .unwrap();
@@ -164,6 +188,10 @@ fn import_chat_attachments_request_response_use_camel_case() {
         request.attachments[0].source_kind,
         Some(ChatAttachmentSourceKind::DragDrop)
     );
+    assert_eq!(
+        request.attachments[1].content_base64.as_deref(),
+        Some("cG5nIGJ5dGVz")
+    );
 
     let response = ImportChatAttachmentsResponseDto {
         attachments: vec![ChatAttachmentDto {