chore(webui): remove timeline animation page

The animated t-SNE 'embedding landscape' added no real analytical value and
its cumulative-by-month logic was inherently confusing (ancient drafts
appearing late in the animation). Removed entirely rather than maintained.

Drops the /timeline route, template, nav link, data builder
(get_timeline_animation_data / _compute_timeline_animation_data) and its test.
The Overview mini-timeline and /api/timeline (separate features) are untouched.

src/webui/templates/base.html

@@ -156,10 +156,6 @@
                 Blog Drafts
             </a>
             {% endif %}
-            <a href="/timeline" class="sidebar-link flex items-center gap-3 px-5 py-2.5 text-sm text-slate-300 {{ 'active' if active_page == 'timeline' }}">
-                <svg class="w-4 h-4 opacity-60" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M12 8v4l3 3m6-3a9 9 0 11-18 0 9 9 0 0118 0z"/></svg>
-                Timeline
-            </a>
             {% if is_admin %}
             <a href="/trends" class="sidebar-link flex items-center gap-3 px-5 py-2.5 text-sm text-slate-300 {{ 'active' if active_page == 'trends' }}">
                 <svg class="w-4 h-4 opacity-60" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M13 7h8m0 0v8m0-8l-8 8-4-4-6 6"/></svg>