feat: multi-select filters for events and channels

- New MultiSelect component with checkbox dropdown
- Event and channel filters now accept multiple selections
- Empty array = all selected (no filter applied)
- URL params store selections as comma-separated values
- District and quarter remain single-select

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/App.css

@@ -762,6 +762,84 @@ table tbody tr:hover {
   border-color: var(--accent);
 }
 
+/* Multi-select */
+.multi-select {
+  position: relative;
+}
+
+.multi-select-trigger {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  width: 100%;
+  padding: 12px 16px;
+  border: 1px solid var(--border);
+  border-radius: 8px;
+  font-size: 0.9375rem;
+  background: var(--surface);
+  color: var(--text-primary);
+  cursor: pointer;
+  text-align: left;
+}
+
+.multi-select-trigger:focus {
+  outline: 2px solid var(--accent);
+  outline-offset: -1px;
+  border-color: var(--accent);
+}
+
+.multi-select-text {
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+  flex: 1;
+}
+
+.multi-select-arrow {
+  font-size: 0.65rem;
+  opacity: 0.5;
+  margin-inline-start: 8px;
+}
+
+.multi-select-dropdown {
+  position: absolute;
+  top: calc(100% + 4px);
+  left: 0;
+  right: 0;
+  background: var(--surface);
+  border: 1px solid var(--border);
+  border-radius: 8px;
+  box-shadow: 0 4px 12px rgba(0,0,0,0.15);
+  z-index: 50;
+  max-height: 240px;
+  overflow-y: auto;
+  padding: 4px;
+}
+
+.multi-select-option {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 8px 12px;
+  border-radius: 6px;
+  cursor: pointer;
+  font-size: 0.875rem;
+  color: var(--text-primary);
+  text-transform: none;
+  letter-spacing: normal;
+  font-weight: normal;
+}
+
+.multi-select-option:hover {
+  background: var(--hover);
+}
+
+.multi-select-option input[type="checkbox"] {
+  width: 16px;
+  height: 16px;
+  accent-color: var(--accent);
+}
+
 .period-display {
   background: var(--bg);
   padding: 16px;