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_feed_sort.py

@@ -0,0 +1,27 @@
+from goodnews.db import connect, init_db
+from goodnews import queries
+
+
+def _article(c, aid, *, when):
+    c.execute(
+        "INSERT INTO articles (id, source_id, canonical_url, title, url_hash, published_at) "
+        "VALUES (?, 1, ?, ?, ?, ?)",
+        (aid, f"http://s/{aid}", f"T{aid}", f"h{aid}", when),
+    )
+    c.execute(
+        "INSERT INTO article_scores (article_id, accepted, constructive_score) VALUES (?, 1, 5)",
+        (aid,),
+    )
+
+
+def test_latest_sorts_strictly_by_recency(tmp_path):
+    c = connect(str(tmp_path / "t.db")); init_db(c)
+    c.execute("INSERT INTO sources (id, name, feed_url) VALUES (1, 'S', 'http://s/f')")
+    # Insert out of order; the dates are what should drive 'latest'.
+    _article(c, 1, when="2026-03-01T00:00:00")
+    _article(c, 2, when="2026-06-01T00:00:00")  # newest
+    _article(c, 3, when="2026-01-01T00:00:00")  # oldest
+    c.commit()
+
+    latest = [a["id"] for a in queries.feed(c, sort="latest")]
+    assert latest == [2, 1, 3]  # newest → oldest, regardless of insert order