fix: bell badge sizing, text contrast, and empty timestamp display

- .tile-bell: inline-flex badge (min-width:16px height:16px) so it's a
  circle for single digits, pill for '9+'; was display:inline so
  width/height were ignored → oval shape
- .tile-bell color: #0D1117 (dark on amber) — was inheriting muted gray
  from .tile-meta, making count illegible
- formatTimestamp(null): return '' instead of em-dash '—' which rendered
  as a confusing glyph in the tile meta area

frontend/style.css

@@ -182,15 +182,23 @@ body {
   margin-left: 8px;
 }
 
-/* Bell dot */
+/* Bell notification badge */
 .tile-bell {
-  width: 8px;
-  height: 8px;
-  border-radius: 50%;
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  min-width: 16px;
+  height: 16px;
+  padding: 0 3px;
+  border-radius: 8px;
   background: var(--bell);
-  animation: bell-pulse 1.4s ease-in-out infinite;
-  margin-right: 6px;
+  color: #0D1117;
+  font-size: 9px;
+  font-weight: 700;
+  line-height: 1;
   flex-shrink: 0;
+  margin-right: 6px;
+  animation: bell-pulse 1.4s ease-in-out infinite;
 }
 
 @keyframes bell-pulse {