Vendor htmx, add Dockerfile, show a placeholder for missing covers

Serve htmx 2.0.4 from the package instead of the jsdelivr CDN. Add a Dockerfile for local testing. Cover images now sit in a styled placeholder box that stays visible when Music Assistant has no cover for an album.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_app.py

@@ -63,6 +63,12 @@ def test_pages_render(client, store):
     assert client.get("/tags/ZZ").status_code == 404
 
 
+def test_htmx_is_served_locally(client):
+    assert 'src="/static/htmx-2.0.4.min.js"' in client.get("/").text
+    r = client.get("/static/htmx-2.0.4.min.js")
+    assert r.status_code == 200 and r.text.startswith("var htmx=")
+
+
 def test_search_results(client, library):
     r = client.get("/tags/AA-BB/search", params={"q": " lateralus "})
     assert library.queries == ["lateralus"]