Add live search progress, session counter, and fix URLs

- Search candidates client-side with JS, showing "Checking X..." spinner
  instead of leaving user waiting on a blank page
- Fix broken api_valid_hit endpoint (get_diff returns dict, not tuple)
- Remove server-side get_best_hit; article_page now returns candidate list
  immediately and JS iterates via /api/1/valid_hit
- URL now reflects current article via history.replaceState (?title=X),
  Skip navigates to ?after=X to advance past it
- Track saves in session; show count as green badge in navbar
- Add session counter incremented on each successful save

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

templates/base.html

@@ -20,6 +20,9 @@
         </form>
         {% if g.user %}
           <span class="text-light small opacity-75">{{ g.user }}</span>
+          {% if session.get("saves") %}
+          <span class="badge bg-success">{{ session["saves"] }} saved</span>
+          {% endif %}
           <a class="btn btn-outline-light btn-sm" href="{{ url_for('oauth_disconnect') }}">Log out</a>
         {% else %}
           <a class="btn btn-outline-light btn-sm" href="{{ url_for('start_oauth') }}">Log in with Wikipedia</a>