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.ts

@@ -32,6 +32,7 @@ import type {
   OpenTerminalOptions,
   ReattachAgentChatResult,
   ReattachResult,
+  SendAgentChatOptions,
   StoppedLiveAgent,
   TerminalHandle,
 } from "@/ports";
@@ -261,12 +262,14 @@ export class TauriAgentGateway implements AgentGateway {
     sessionId: string,
     prompt: string,
     onChunk: (chunk: ReplyChunk) => void,
+    options: SendAgentChatOptions = {},
   ): Promise<void> {
     const channel = new Channel<ReplyChunk>();
     channel.onmessage = onChunk;
     await invoke("agent_send", {
       sessionId,
       prompt,
+      ...(options.attachments ? { attachments: options.attachments } : {}),
       onReply: channel,
     });
   }