feat(gamification): list_recent_completions_cmd Tauri wrapper

Thin wrapper over kon_storage::list_recent_completions, parameterised
by day count. Serialises to camelCase JSON (day, count).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src-tauri/src/lib.rs

@@ -317,6 +317,7 @@ pub fn run() {
             commands::tasks::extract_tasks_from_transcript_cmd,
             commands::tasks::list_subtasks_cmd,
             commands::tasks::complete_subtask_cmd,
+            commands::tasks::list_recent_completions_cmd,
             // HITL feedback (Phase 2 roadmap)
             commands::feedback::record_feedback,
             commands::feedback::list_feedback_examples_cmd,