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/backend/src/dto.rs

@@ -2971,7 +2971,14 @@ impl From<TerminalSession> for TerminalSessionDto {
 #[serde(rename_all = "camelCase")]
 pub struct ChatAttachmentInputDto {
     /// Local source path selected or staged by the driving adapter.
-    pub path: String,
+    #[serde(default)]
+    pub path: Option<String>,
+    /// Display filename for in-memory content, for example clipboard images.
+    #[serde(default)]
+    pub filename: Option<String>,
+    /// Base64-encoded raw content for in-memory attachment import.
+    #[serde(default)]
+    pub content_base64: Option<String>,
     /// Optional MIME type known by the frontend/OS picker.
     #[serde(default)]
     pub mime: Option<String>,