feat(stage): show Bandcamp link in ON AIR

tests/test_app.py

@@ -133,3 +133,27 @@ async def test_narrow_layout() -> None:
         text = rendered_text(app)
         assert "STATIONS" in text
         assert "ON AIR" in text
+
+
+@pytest.mark.asyncio
+async def test_stage_shows_bandcamp_link_only_when_present() -> None:
+    app = SomaFMApp(autoplay=False, offline=True, audio=False)
+    async with app.run_test(size=(110, 40)) as pilot:
+        await pilot.pause()
+        station = app.current
+        assert station is not None
+        song = Song(
+            title="Rauschebart",
+            artist="Jens Buchert",
+            album="Gravity",
+            bandcamp_url="https://jensbuchert.bandcamp.com/track/rauschebart",
+        )
+        station.now_playing = song
+        app.query_one(StagePanel).refresh_stage()
+        await pilot.pause()
+        assert "Bandcamp" in rendered_text(app)
+
+        song.bandcamp_url = ""
+        app.query_one(StagePanel).refresh_stage()
+        await pilot.pause()
+        assert "Bandcamp" not in rendered_text(app)