feat(brief): locked sections, list formatting, auto/manual placeholder chips

- brief.machine: reducer refuses edits to locked (predefined) sections as
  defense-in-depth; LetterSection gains a `locked` flag
- rich-text: paragraphs gain optional `list` kind; editor gets bullet/numbered
  list buttons, keyboard shortcuts, and backspace-deletes-adjacent-chip
- placeholder chips distinguish auto-resolvable (grey) vs manual (yellow), in
  both the editor and the read-only preview
- fix: preview chip now renders matching {…} braces (was a one-sided ⌗ glyph),
  aligned with the editor's chip styling

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/shared/infrastructure/api-client.ts

@@ -1207,6 +1207,42 @@ export class ApiClient {
         }
         return Promise.resolve<BriefDto>(null as any);
     }
+
+    /**
+     * @return OK
+     */
+    briefReset(): Promise<BriefViewDto> {
+        let url_ = this.baseUrl + "/api/v1/brief/reset";
+        url_ = url_.replace(/[?&]$/, "");
+
+        let options_: RequestInit = {
+            method: "POST",
+            headers: {
+                "Accept": "application/json"
+            }
+        };
+
+        return this.http.fetch(url_, options_).then((_response: Response) => {
+            return this.processBriefReset(_response);
+        });
+    }
+
+    protected processBriefReset(response: Response): Promise<BriefViewDto> {
+        const status = response.status;
+        let _headers: any = {}; if (response.headers && response.headers.forEach) { response.headers.forEach((v: any, k: any) => _headers[k] = v); };
+        if (status === 200) {
+            return response.text().then((_responseText) => {
+            let result200: any = null;
+            result200 = _responseText === "" ? null : JSON.parse(_responseText, this.jsonParseReviver) as BriefViewDto;
+            return result200;
+            });
+        } else if (status !== 200 && status !== 204) {
+            return response.text().then((_responseText) => {
+            return throwException("An unexpected server error occurred.", status, _responseText, _headers);
+            });
+        }
+        return Promise.resolve<BriefViewDto>(null as any);
+    }
 }
 
 export interface AantekeningDto {
@@ -1369,6 +1405,7 @@ export interface LetterSectionDto {
     title?: string | undefined;
     required?: boolean;
     blocks?: LetterBlockDto[] | undefined;
+    locked?: boolean;
 }
 
 export interface LibraryPassageDto {
@@ -1388,6 +1425,7 @@ export interface ManualDiplomaPolicyDto {
 
 export interface ParagraphDto {
     nodes?: RichTextNodeDto[] | undefined;
+    list?: string | undefined;
 }
 
 export interface PersonDto {