add abandoned status to distinguish given-up habits from completed ones

Completed and abandoned habits were both lumped under a single terminal
state, making it impossible to tell "finished successfully" apart from
"gave up" when reviewing habit history.

app/templates/habits.html

@@ -8,6 +8,7 @@
   <a href="/habits?tab=build" class="{{ 'active' if tab == 'build' }}">만들고 싶은 습관</a>
   <a href="/habits?tab=quit" class="{{ 'active' if tab == 'quit' }}">멈추고 싶은 습관</a>
   <a href="/habits?tab=completed" class="{{ 'active' if tab == 'completed' }}">완료된 습관</a>
+  <a href="/habits?tab=abandoned" class="{{ 'active' if tab == 'abandoned' }}">포기한 습관</a>
 </div>
 
 {% if tab in ("build", "quit") %}
@@ -25,6 +26,7 @@
     <div class="empty-state">
       {% if tab == "build" %}만들고 싶은 습관이 아직 없어요. 위에서 추가해보세요.
       {% elif tab == "quit" %}끊고 싶은 습관이 아직 없어요. 위에서 추가해보세요.
+      {% elif tab == "abandoned" %}아직 포기한 습관이 없어요.
       {% else %}아직 완료된 습관이 없어요.{% endif %}
     </div>
   {% endif %}