feat(browse): sticky tabs + per-tab search bar (server-side, scope-aware)

The Browse tab nav scrolled away (operator didn't know it existed) and
Posts had no search. Roll the tab strip + a shared search field into one
sticky block pinned under the 64px TopNav.

- Posts gains server-side text search: PostFeedService.scroll()/around()
  + /api/posts accept q (ILIKE over post_title OR description), applied
  INSIDE the artist/platform WHERE so search stays scoped to the active
  filter. Scope shown as clearable chips next to the search field.
- Artists/Tags search consolidates into the sticky bar: their inner
  search boxes are removed; they react to route.query.q (q is deep-
  linkable, e.g. /browse?tab=posts&q=foo). Platform/kind filters stay.
- Posts empty state now distinguishes 'no matches' from 'no posts yet'.

Tests: posts q-search matches title|description and stays artist-scoped
(service); q passthrough (api).

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

tests/test_api_posts.py

@@ -88,6 +88,16 @@ async def test_list_filter_propagates_artist(client, seeded_post):
     assert body["items"][0]["id"] == post.id
 
 
+@pytest.mark.asyncio
+async def test_list_text_search_propagates(client, seeded_post):
+    _, _, post = seeded_post  # title "Hello", description "<p>hi</p>"
+    hit = await client.get("/api/posts?q=hello")
+    assert hit.status_code == 200
+    assert [it["id"] for it in (await hit.get_json())["items"]] == [post.id]
+    miss = await client.get("/api/posts?q=zzznope")
+    assert (await miss.get_json())["items"] == []
+
+
 @pytest.mark.asyncio
 async def test_detail_200_for_known(client, seeded_post):
     _, _, post = seeded_post

tests/test_post_feed_service.py

@@ -180,6 +180,54 @@ async def test_scroll_filters_by_platform(db):
     assert [it["id"] for it in page["items"]] == [pp.id]
 
 
+@pytest.mark.asyncio
+async def test_scroll_text_search_matches_title_or_description(db):
+    artist = await _seed_artist(db, "alice-search")
+    src = await _seed_source(db, artist.id, "patreon", "https://p/alice-se")
+    now = datetime.now(UTC)
+    by_title = await _seed_post(
+        db, src.id, external_id="T", post_date=now,
+        title="Dragon Quest cover", description="<p>nothing</p>",
+    )
+    by_desc = await _seed_post(
+        db, src.id, external_id="D", post_date=now - timedelta(days=1),
+        title="Untitled", description="<p>a sleeping dragon</p>",
+    )
+    await _seed_post(
+        db, src.id, external_id="N", post_date=now - timedelta(days=2),
+        title="Cat nap", description="<p>just a cat</p>",
+    )
+    await db.commit()
+
+    page = await PostFeedService(db).scroll(cursor=None, limit=10, q="dragon")
+    ids = {it["id"] for it in page["items"]}
+    # ILIKE is case-insensitive and spans title OR description; the cat post
+    # matches neither.
+    assert ids == {by_title.id, by_desc.id}
+
+
+@pytest.mark.asyncio
+async def test_scroll_text_search_stays_scoped_to_artist(db):
+    alice = await _seed_artist(db, "alice-scope")
+    bob = await _seed_artist(db, "bob-scope")
+    src_a = await _seed_source(db, alice.id, "patreon", "https://p/alice-sc")
+    src_b = await _seed_source(db, bob.id, "patreon", "https://p/bob-sc")
+    now = datetime.now(UTC)
+    alice_hit = await _seed_post(
+        db, src_a.id, external_id="AH", post_date=now, title="comet study",
+    )
+    # Same query word, different artist — must be excluded by the scope.
+    await _seed_post(
+        db, src_b.id, external_id="BH", post_date=now, title="comet sketch",
+    )
+    await db.commit()
+
+    page = await PostFeedService(db).scroll(
+        cursor=None, limit=10, q="comet", artist_id=alice.id,
+    )
+    assert [it["id"] for it in page["items"]] == [alice_hit.id]
+
+
 @pytest.mark.asyncio
 async def test_scroll_combined_artist_and_platform(db):
     alice = await _seed_artist(db, "alice-combo")