fix(ui): white text on download + link buttons (was black-on-black)

The button CSS only styled .stButton — download buttons (.stDownloadButton)
and link buttons (.stLinkButton) rendered with dark default text on dark
background, invisible. Added them to the gradient-button rule and forced
white text on all nested label elements (Streamlit wraps labels in p/span
that inherit dark color). Kept secondary/ghost buttons blue-on-white by
overriding the white-children rule for .secondary-btn.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

ui.py

@@ -59,32 +59,46 @@ p, span, label, .stMarkdown { color: #0F172A; }
     color: #334155 !important; font-weight: 500 !important;
 }
 
-/* ── Buttons ── */
-.stButton > button[kind="primary"], .stButton > button {
+/* ── Buttons (regular + download + link) ── */
+.stButton > button[kind="primary"], .stButton > button,
+.stDownloadButton > button,
+.stLinkButton > a, .stLinkButton > a[data-testid] {
     background: linear-gradient(135deg, #2563EB 0%, #7C3AED 100%) !important;
-    color: white !important; border: none !important; border-radius: 10px !important;
+    color: #ffffff !important; border: none !important; border-radius: 10px !important;
     padding: 10px 24px !important; font-weight: 600 !important;
     font-family: 'Inter', sans-serif !important;
     transition: all 0.2s ease !important;
     box-shadow: 0 2px 8px rgba(37,99,235,0.25) !important;
+    text-decoration: none !important;
 }
-.stButton > button:hover {
+/* Force white text on every nested element inside these buttons
+   (Streamlit wraps the label in <p>/<span>/<div> that inherit dark text) */
+.stButton > button *, .stDownloadButton > button *, .stLinkButton > a * {
+    color: #ffffff !important;
+    fill: #ffffff !important;
+}
+.stButton > button:hover, .stDownloadButton > button:hover, .stLinkButton > a:hover {
     transform: translateY(-1px) !important;
     box-shadow: 0 4px 16px rgba(37,99,235,0.35) !important;
+    color: #ffffff !important;
 }
-.stButton > button:disabled {
+.stButton > button:disabled, .stDownloadButton > button:disabled {
     opacity: 0.5 !important;
     transform: none !important;
     box-shadow: none !important;
 }
 
-/* Secondary / ghost buttons */
+/* Secondary / ghost buttons — white bg, blue text (override the white-children rule) */
 .secondary-btn .stButton > button {
     background: white !important;
     color: #2563EB !important;
     border: 1.5px solid #E2E8F0 !important;
     box-shadow: 0 1px 3px rgba(0,0,0,0.04) !important;
 }
+.secondary-btn .stButton > button *,
+.secondary-btn .stButton > button:hover * {
+    color: #2563EB !important;
+}
 .secondary-btn .stButton > button:hover {
     border-color: #2563EB !important;
     background: #F0F4FF !important;