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.

tests/test_web.py

@@ -61,11 +61,6 @@ def test_authors_page(client):
     assert resp.status_code == 200
 
 
-def test_timeline_page(client):
-    resp = client.get("/timeline")
-    assert resp.status_code == 200
-
-
 def test_search_page_empty(client):
     resp = client.get("/search")
     assert resp.status_code == 200