feat: quick actions on email rows (read/unread, star, trash)

- EmailRow shared component with hover action buttons: mark read/unread, star/unstar, trash
- Optimistic UI — local state patches immediately on click; row fades and becomes non-interactive during the API call
- Trashed rows disappear from the current folder view via onRemove callback
- Backend: EmailController single-email endpoints (POST /email/{id}/read|unread|star|unstar|trash|untrash)
- Star/Unstar added to CleanupActionType enum and CleanupService (maps to STARRED Gmail label via BatchModifyAsync)
- UserId scope enforced in ResolveTargetsAsync — a user can only act on their own emails
- action buttons use stopPropagation so clicking them does not open Gmail

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/api/client.js

@@ -98,6 +98,15 @@ function folderToRequest(slug, page, pageSize) {
   }
 }
 
+export const EmailApi = {
+  markRead:   (id) => api.post(`/email/${id}/read`),
+  markUnread: (id) => api.post(`/email/${id}/unread`),
+  star:       (id) => api.post(`/email/${id}/star`),
+  unstar:     (id) => api.post(`/email/${id}/unstar`),
+  trash:      (id) => api.post(`/email/${id}/trash`),
+  untrash:    (id) => api.post(`/email/${id}/untrash`),
+};
+
 export const ExportApi = {
   reportUrl: (format) => `/api/v1/export/report?format=${format}`
 };