Introduce MiniMessage rendering system for structured chat formatting and interaction

frontend/src/app/pages/altitude/chat/mini-message/mini-message-interaction.service.ts

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+import { Injectable } from '@angular/core';
+import { Subject } from 'rxjs';
+import { MiniMessageClickPayload } from './mini-message.types';
+
+/**
+ * Because <mini-message> renders itself recursively for `extra` children, a plain
+ * @Output() would need to be manually re-emitted at every level. Instead, every
+ * instance injects this single service (provided once at the root usage) and pushes
+ * clicks through it. Subscribe to `clicks$` wherever you render the top-level component.
+ */
+@Injectable()
+export class MiniMessageInteractionService {
+  private readonly clickSubject = new Subject<MiniMessageClickPayload>();
+  readonly clicks$ = this.clickSubject.asObservable();
+
+  emitClick(payload: MiniMessageClickPayload): void {
+    this.clickSubject.next(payload);
+  }
+}