feat(page-tree): gate compact tree density behind COMPACT_PAGE_TREE flag

Make the denser page-tree layout opt-in instead of hardcoded, so row
density can be toggled per deployment via the COMPACT_PAGE_TREE runtime
config flag.

- doc-tree: extract ROW_HEIGHT_STANDARD (32) / ROW_HEIGHT_COMPACT (26);
  default the virtualizer row stride to STANDARD density.
- client: isCompactPageTreeEnabled() in lib/config (reads
  COMPACT_PAGE_TREE, default true); used by space-tree and shared-tree
  to choose the row height.
- server: EnvironmentService.isCompactPageTreeEnabled() and expose
  COMPACT_PAGE_TREE through the window runtime config (static.module).

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

apps/server/src/integrations/environment/environment.service.ts

@@ -214,6 +214,13 @@ export class EnvironmentService {
     return !this.isCloud();
   }
 
+  isCompactPageTreeEnabled(): boolean {
+    const compactTree = this.configService
+      .get<string>('COMPACT_PAGE_TREE', 'true')
+      .toLowerCase();
+    return compactTree === 'true';
+  }
+
   getStripePublishableKey(): string {
     return this.configService.get<string>('STRIPE_PUBLISHABLE_KEY');
   }