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/components/shared/MultiSelect.tsx

@@ -0,0 +1,82 @@
+import React, { useState, useRef, useEffect } from 'react';
+
+interface MultiSelectProps {
+  options: string[];
+  selected: string[];
+  onChange: (selected: string[]) => void;
+  allLabel: string;
+  placeholder?: string;
+}
+
+function MultiSelect({ options, selected, onChange, allLabel, placeholder }: MultiSelectProps) {
+  const [open, setOpen] = useState(false);
+  const ref = useRef<HTMLDivElement>(null);
+
+  // Close on outside click
+  useEffect(() => {
+    const handleClick = (e: MouseEvent) => {
+      if (ref.current && !ref.current.contains(e.target as Node)) {
+        setOpen(false);
+      }
+    };
+    document.addEventListener('mousedown', handleClick);
+    return () => document.removeEventListener('mousedown', handleClick);
+  }, []);
+
+  const isAll = selected.length === 0;
+
+  const toggle = (value: string) => {
+    if (selected.includes(value)) {
+      onChange(selected.filter(v => v !== value));
+    } else {
+      onChange([...selected, value]);
+    }
+  };
+
+  const selectAll = () => onChange([]);
+
+  const displayText = isAll
+    ? allLabel
+    : selected.length === 1
+      ? selected[0]
+      : `${selected.length} selected`;
+
+  return (
+    <div className="multi-select" ref={ref}>
+      <button
+        type="button"
+        className="multi-select-trigger"
+        onClick={() => setOpen(!open)}
+        aria-expanded={open}
+      >
+        <span className="multi-select-text">{displayText}</span>
+        <span className="multi-select-arrow">{open ? '▲' : '▼'}</span>
+      </button>
+
+      {open && (
+        <div className="multi-select-dropdown">
+          <label className="multi-select-option">
+            <input
+              type="checkbox"
+              checked={isAll}
+              onChange={selectAll}
+            />
+            <span>{allLabel}</span>
+          </label>
+          {options.map(opt => (
+            <label key={opt} className="multi-select-option">
+              <input
+                type="checkbox"
+                checked={selected.includes(opt)}
+                onChange={() => toggle(opt)}
+              />
+              <span>{opt}</span>
+            </label>
+          ))}
+        </div>
+      )}
+    </div>
+  );
+}
+
+export default MultiSelect;

src/components/shared/index.tsx

@@ -2,5 +2,6 @@ export { default as Carousel } from './Carousel';
 export { default as ChartCard } from './ChartCard';
 export { default as EmptyState } from './EmptyState';
 export { default as FilterControls } from './FilterControls';
+export { default as MultiSelect } from './MultiSelect';
 export { default as StatCard } from './StatCard';
 export { default as ToggleSwitch } from './ToggleSwitch';