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/tests/test_app.mjs

@@ -62,8 +62,8 @@ test('app.js exports all 7 pure functions', () => {
 
 // --- formatTimestamp ---
 
-test('formatTimestamp returns em-dash for null', () => {
-  assert.strictEqual(app.formatTimestamp(null), '\u2014');
+test('formatTimestamp returns empty string for null', () => {
+  assert.strictEqual(app.formatTimestamp(null), '');
 });
 
 test('formatTimestamp returns seconds ago for timestamp < 60s ago', () => {