perf: memoize TranslatePipe instead of re-walking translations every CD cycle

pure:false stays (needed so language switches propagate without
touching every | translate template call site), but repeat calls with
unchanged key/params/language now hit a Map lookup instead of
re-splitting the key and re-walking the translation object tree.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/i18n/translate.pipe.ts

@@ -1,14 +1,42 @@
 import { Pipe, PipeTransform, inject } from '@angular/core';
 import { TranslateService } from './translate.service';
+import { LanguageService } from '../services/language.service';
 
+/**
+ * Stays impure (must re-run every CD cycle so a live language switch
+ * updates every binding without touching hundreds of `| translate`
+ * template call sites to pass the language explicitly as a pure-pipe
+ * argument). Memoized per-instance instead: repeat calls with the same
+ * key/params/language - the overwhelming majority of CD cycles, since
+ * nothing actually changed - hit a Map lookup instead of re-walking the
+ * translation tree and re-running the interpolation regex.
+ */
 @Pipe({
   name: 'translate',
   pure: false,
 })
 export class TranslatePipe implements PipeTransform {
   private translateService = inject(TranslateService);
+  private langService = inject(LanguageService);
+
+  private lastLang = '';
+  private readonly cache = new Map<string, string>();
 
   transform(key: string, params?: Record<string, string | number>): string {
-    return this.translateService.t(key, params);
+    const lang = this.langService.currentLanguage();
+    if (lang !== this.lastLang) {
+      this.lastLang = lang;
+      this.cache.clear();
+    }
+
+    const cacheKey = params ? `${key}::${JSON.stringify(params)}` : key;
+    const cached = this.cache.get(cacheKey);
+    if (cached !== undefined) {
+      return cached;
+    }
+
+    const result = this.translateService.t(key, params);
+    this.cache.set(cacheKey, result);
+    return result;
   }
 }