refactor(tree): borrow cleanups from the sibling expand-all impl

- extract collectAllIds / collectBranchIds into tree/utils and use them in
  space-tree.tsx instead of inline closures
- drop the duplicate SidebarPageTreeDto, reuse the existing SidebarPageDto
  for the /pages/tree endpoint
- type the getSpaceTree client call as api.post<{ items: IPage[] }>

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

apps/server/src/core/page/dto/sidebar-page.dto.ts

@@ -9,13 +9,3 @@ export class SidebarPageDto {
   @IsString()
   pageId: string;
 }
-
-export class SidebarPageTreeDto {
-  @IsOptional()
-  @IsUUID()
-  spaceId?: string;
-
-  @IsOptional()
-  @IsString()
-  pageId?: string;
-}

apps/server/src/core/page/page.controller.ts

@@ -32,7 +32,7 @@ import {
 import { JwtAuthGuard } from '../../common/guards/jwt-auth.guard';
 import { PaginationOptions } from '@docmost/db/pagination/pagination-options';
 import { Page, User, Workspace } from '@docmost/db/types/entity.types';
-import { SidebarPageDto, SidebarPageTreeDto } from './dto/sidebar-page.dto';
+import { SidebarPageDto } from './dto/sidebar-page.dto';
 import {
   SpaceCaslAction,
   SpaceCaslSubject,
@@ -581,7 +581,7 @@ export class PageController {
   @HttpCode(HttpStatus.OK)
   @Post('/tree')
   async getPagesTree(
-    @Body() dto: SidebarPageTreeDto,
+    @Body() dto: SidebarPageDto,
     @AuthUser() user: User,
   ) {
     if (!dto.spaceId && !dto.pageId) {