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

frontend/src/adapters/agent.test.ts

@@ -171,6 +171,38 @@ describe("TauriAgentGateway invoke payloads", () => {
     expect(invoke).not.toHaveBeenCalledWith("close_agent_session", expect.anything());
   });
 
+  it("sendAgentChat forwards structured clipboard attachments to agent_send", async () => {
+    await new TauriAgentGateway().sendAgentChat(
+      "chat-session-1",
+      "",
+      vi.fn(),
+      {
+        attachments: [
+          {
+            filename: "clipboard.png",
+            contentBase64: "AQID",
+            mime: "image/png",
+            sourceKind: "clipboard",
+          },
+        ],
+      },
+    );
+
+    expect(invoke).toHaveBeenCalledWith("agent_send", {
+      sessionId: "chat-session-1",
+      prompt: "",
+      attachments: [
+        {
+          filename: "clipboard.png",
+          contentBase64: "AQID",
+          mime: "image/png",
+          sourceKind: "clipboard",
+        },
+      ],
+      onReply: expect.anything(),
+    });
+  });
+
   it("launchAgentChat returns a handle only when launch_agent confirms cellKind chat", async () => {
     invoke.mockResolvedValueOnce({
       sessionId: "chat-session-1",