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/src/commands.rs

@@ -4,7 +4,7 @@
 //! [`AppState`], map `Result<Output, AppError>` to `Result<ResponseDto,
 //! ErrorDto>`. No business logic lives here.
 
-use base64::engine::general_purpose::URL_SAFE_NO_PAD;
+use base64::engine::general_purpose::{STANDARD, STANDARD_NO_PAD, URL_SAFE, URL_SAFE_NO_PAD};
 use base64::Engine;
 use serde::{Deserialize, Serialize};
 use tauri::ipc::Channel;
@@ -2326,22 +2326,17 @@ async fn import_chat_attachments_for_session(
     for path in legacy_paths {
         if seen_paths.insert(path.clone()) {
             items.push(application::ImportChatAttachmentItem {
-                path,
+                source: application::ImportChatAttachmentSource::Path(path),
                 mime: None,
                 source_kind: domain::ChatAttachmentSourceKind::LocalFile,
             });
         }
     }
     for attachment in attachments {
-        if seen_paths.insert(attachment.path.clone()) {
-            items.push(application::ImportChatAttachmentItem {
-                path: attachment.path,
-                mime: attachment.mime,
-                source_kind: attachment
-                    .source_kind
-                    .unwrap_or(domain::ChatAttachmentSourceKind::LocalFile),
-            });
-        }
+        let Some(item) = chat_attachment_input_to_item(attachment, &mut seen_paths)? else {
+            continue;
+        };
+        items.push(item);
     }
 
     let output = state
@@ -2361,6 +2356,71 @@ async fn import_chat_attachments_for_session(
         .collect())
 }
 
+fn chat_attachment_input_to_item(
+    attachment: ChatAttachmentInputDto,
+    seen_paths: &mut std::collections::HashSet<String>,
+) -> Result<Option<application::ImportChatAttachmentItem>, ErrorDto> {
+    let has_path = attachment
+        .path
+        .as_deref()
+        .is_some_and(|path| !path.is_empty());
+    let has_content = attachment
+        .content_base64
+        .as_deref()
+        .is_some_and(|content| !content.is_empty());
+    if has_path && has_content {
+        return Err(ErrorDto::from(AppError::Invalid(
+            "chat attachment must provide either path or contentBase64, not both".to_owned(),
+        )));
+    }
+    if let Some(path) = attachment.path.filter(|path| !path.is_empty()) {
+        if !seen_paths.insert(path.clone()) {
+            return Ok(None);
+        }
+        return Ok(Some(application::ImportChatAttachmentItem {
+            source: application::ImportChatAttachmentSource::Path(path),
+            mime: attachment.mime,
+            source_kind: attachment
+                .source_kind
+                .unwrap_or(domain::ChatAttachmentSourceKind::LocalFile),
+        }));
+    }
+    if let Some(content_base64) = attachment
+        .content_base64
+        .filter(|content| !content.is_empty())
+    {
+        let filename = attachment.filename.ok_or_else(|| {
+            ErrorDto::from(AppError::Invalid(
+                "chat attachment bytes require filename".to_owned(),
+            ))
+        })?;
+        let bytes = decode_attachment_content_base64(&content_base64)?;
+        return Ok(Some(application::ImportChatAttachmentItem {
+            source: application::ImportChatAttachmentSource::Bytes { filename, bytes },
+            mime: attachment.mime,
+            source_kind: attachment
+                .source_kind
+                .unwrap_or(domain::ChatAttachmentSourceKind::Clipboard),
+        }));
+    }
+    Err(ErrorDto::from(AppError::Invalid(
+        "chat attachment must provide path or contentBase64".to_owned(),
+    )))
+}
+
+fn decode_attachment_content_base64(raw: &str) -> Result<Vec<u8>, ErrorDto> {
+    STANDARD
+        .decode(raw)
+        .or_else(|_| STANDARD_NO_PAD.decode(raw))
+        .or_else(|_| URL_SAFE.decode(raw))
+        .or_else(|_| URL_SAFE_NO_PAD.decode(raw))
+        .map_err(|_| {
+            ErrorDto::from(AppError::Invalid(
+                "invalid chat attachment contentBase64".to_owned(),
+            ))
+        })
+}
+
 fn prompt_with_imported_attachments(prompt: &str, attachments: &[ChatAttachmentDto]) -> String {
     if attachments.is_empty() {
         return prompt.to_owned();