fix(ui): mobile viewport, scrollable popovers, and actor labels

- Set viewport-fit=cover and disable user scaling for mobile PWA feel
- Wrap assignee/project popover lists in scrollable containers
- Remove rounded-t-sm from stacked chart bars for cleaner rendering
- Prevent filter bar icons from shrinking on narrow screens
- Show "Board" instead of raw user IDs in activity feeds and toasts
- Surface server error message in health API failures

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/api/health.ts

@@ -13,7 +13,8 @@ export const healthApi = {
       headers: { Accept: "application/json" },
     });
     if (!res.ok) {
-      throw new Error(`Failed to load health (${res.status})`);
+      const payload = await res.json().catch(() => null) as { error?: string } | null;
+      throw new Error(payload?.error ?? `Failed to load health (${res.status})`);
     }
     return res.json();
   },