feat(web): use a range calendar picker for fan dates

reaction_board/web/static/style.css

@@ -531,24 +531,101 @@ h1, h2, h3, p { margin: 0; }
   .filter-row .select { width: 100%; max-width: none; }
 }
 .growth-days { padding-top: 2px; }
-.date-range {
+.fans-range {
   display: flex;
   flex-wrap: wrap;
   align-items: center;
-  gap: 6px;
+  gap: 8px;
   margin-top: 8px;
 }
-.date-range input[type="date"] {
+.fans-range .chip-row { flex: 0 0 auto; min-width: 0; }
+.fan-range-picker { position: relative; display: flex; align-items: center; gap: 6px; }
+@media (min-width: 900px) {
+  .fans-range { flex-wrap: nowrap; }
+  .fans-range .chip-row { flex: 1 1 auto; }
+  .fan-range-picker { flex: 0 0 auto; }
+}
+@media (max-width: 899px) {
+  .fans-range .chip-row { flex-basis: 100%; }
+}
+.date-range-trigger {
   min-height: 38px;
-  padding: 8px 11px;
+  padding: 8px 12px;
   border: 1px solid var(--border);
   border-radius: 10px;
   background: var(--surface);
   color: inherit;
   font: inherit;
   font-size: 13px;
+  white-space: nowrap;
+  cursor: pointer;
 }
-.date-range span { color: var(--muted); font-size: 12px; }
+.date-range-trigger:hover { border-color: var(--accent); color: var(--accent); }
+.range-popover {
+  position: absolute;
+  z-index: 50;
+  top: calc(100% + 6px);
+  left: 0;
+  width: min(300px, calc(100vw - 24px));
+  padding: 12px;
+  border: 1px solid var(--border);
+  border-radius: 14px;
+  background: var(--surface);
+  box-shadow: var(--shadow);
+}
+.range-popover-head {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 8px;
+  margin-bottom: 8px;
+}
+.range-popover-head strong { font-size: 13px; }
+.range-nav {
+  width: 30px;
+  height: 30px;
+  border: 1px solid var(--border);
+  border-radius: 8px;
+  background: transparent;
+  color: var(--text);
+  font-size: 16px;
+  line-height: 1;
+  cursor: pointer;
+}
+.range-weekdays,
+.range-grid {
+  display: grid;
+  grid-template-columns: repeat(7, 1fr);
+  gap: 2px;
+}
+.range-weekdays {
+  margin-bottom: 4px;
+  color: var(--muted);
+  font-size: 11px;
+  text-align: center;
+}
+.range-empty { height: 32px; }
+.range-day {
+  height: 32px;
+  border: 1px solid transparent;
+  border-radius: 8px;
+  background: transparent;
+  color: var(--text);
+  font-size: 12px;
+  cursor: pointer;
+}
+.range-day:hover { background: var(--surface-2); }
+.range-day.in-range { background: var(--accent-soft); color: var(--accent-strong); }
+.range-day.is-start,
+.range-day.is-end { background: var(--accent); color: var(--accent-contrast); font-weight: 650; }
+.range-popover-foot {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 8px;
+  margin-top: 10px;
+}
+.range-hint { color: var(--muted); font-size: 11.5px; }
 
 .rank-list { display: flex; flex-direction: column; gap: 8px; }
 .pager {