Add a permanent "Latest" lane beside "Highlights"

Restructure the nav around two permanent lanes, then the reader's chosen ones:
"Highlights" (the curated daily brief — formerly "Today") and "Latest" (the
freshest accepted stories, newest-first). Now that the gate is tight, a
chronological "incoming" feed is safe to expose.

* feed(): new sort="latest" (pure recency) alongside the default best-first
  rank; /api/feed exposes sort=ranked|latest (validated). Still accepted-only
  and boundary-respecting either way.
* lanes.py: two pinned lanes (Highlights + Latest) instead of one.
* Home: "Latest" view + "Load more" pagination for every feed view (offset-
  paged, de-duped). Mobile bottom bar gains a Latest tab.
* LanePicker shows both pinned lanes; nav rail renders them first.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_lanes.py

@@ -3,12 +3,13 @@ from goodnews.lanes import build_lane_pool, known_lane_keys, DEFAULT_LANES
 
 def test_pool_shape_and_pinned():
     pool = build_lane_pool({"science": 100}, {"space": 200, "food": 0, "innovation": 99})
-    assert pool["pinned"]["key"] == "today"
+    pinned_keys = [p["key"] for p in pool["pinned"]]
+    assert pinned_keys == ["today", "latest"]  # Highlights + Latest, always first
     assert pool["default"] == DEFAULT_LANES
     names = [g["name"] for g in pool["groups"]]
     assert names == ["Moods", "Topics", "Discovery"]
-    # 'today' is pinned, never part of the selectable pool.
-    assert "today" not in known_lane_keys(pool)
+    # pinned lanes are never part of the selectable pool.
+    assert "today" not in known_lane_keys(pool) and "latest" not in known_lane_keys(pool)
 
 
 def test_topics_are_bare_keys_tags_are_prefixed():