feat: fix signature editor init and expand README

- Pass initialContent prop to WysiwygEditor instead of imperative setContent
  to ensure mail signature loads correctly on mount
- Rewrite README to reflect full project scope (scanner inbox, email import,
  postprocessing rules, label printing, OIDC auth)

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

paperless-frontend/src/components/WysiwygEditor.tsx

@@ -26,6 +26,7 @@ export interface WysiwygEditorHandle {
 
 interface WysiwygEditorProps {
   minHeight?: number;
+  initialContent?: string;
 }
 
 function ToolBtn({
@@ -69,14 +70,14 @@ const HEADING_OPTIONS = [
 ];
 
 export const WysiwygEditor = forwardRef<WysiwygEditorHandle, WysiwygEditorProps>(
-  ({ minHeight = 200 }, ref) => {
+  ({ minHeight = 200, initialContent }, ref) => {
     const editor = useEditor({
       extensions: [
         StarterKit,
         Underline,
         TextAlign.configure({ types: ['heading', 'paragraph'] }),
       ],
-      content: '',
+      content: initialContent ?? '',
     });
 
     useImperativeHandle(