fix(P2-10): is_placeholder now reflects actual LLM driver availability

Previously hardcoded to false in Tauri bridge. Now checks whether
kernel provided a driver before building classroom, correctly flagging
placeholder content when LLM is unavailable.

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

desktop/src/types/classroom.ts

@@ -109,6 +109,8 @@ export interface ClassroomMetadata {
   sourceDocument?: string;
   model?: string;
   version: string;
+  /** P2-10: marks content generated by placeholder fallback when LLM fails */
+  is_placeholder?: boolean;
   custom: Record<string, unknown>;
 }