feat(ui): thread optional language override through meetings.reprocess (T8.7, M4.2)

src/lib/stores/meetings.svelte.ts

@@ -218,9 +218,11 @@ class MeetingsStore {
     if (this.selectedId === id) await this.select(id);
   }
 
-  /** Batch re-transcribe with a different (typically larger) model (T3.8). */
-  async reprocess(id: MeetingId, model: string) {
-    await api.reprocessTranscript(id, model);
+  /** Batch re-transcribe with a different (typically larger) model (T3.8).
+   * `language` (T8.7, FR-TRX-4): omitted reuses the meeting's current
+   * language rather than resetting it to auto. */
+  async reprocess(id: MeetingId, model: string, language?: string) {
+    await api.reprocessTranscript(id, model, language);
     await this.load();
     if (this.selectedId === id) await this.select(id);
   }