polish: fix page title, multi-select styling, chart colors

- Page title: "HiHala Data - Museums" -> "HiHala Data"
- Meta description: updated to "Event analytics"
- Multi-select dropdown: fix inherited uppercase, wider to fit labels
- Multi-select arrow: smooth CSS rotation instead of swapping characters
- Chart colors: 10-color palette for events/channels (was 3)
- Remove unused ArcElement (Doughnut) from Chart.js registration (-5KB)
- District chart uses dynamic palette instead of hardcoded 2 colors

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

src/components/shared/MultiSelect.tsx

@@ -50,7 +50,7 @@ function MultiSelect({ options, selected, onChange, allLabel, placeholder }: Mul
         aria-expanded={open}
       >
         <span className="multi-select-text">{displayText}</span>
-        <span className="multi-select-arrow">{open ? '▲' : '▼'}</span>
+        <span className="multi-select-arrow">▼</span>
       </button>
 
       {open && (