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/components/IssuesList.tsx

@@ -214,7 +214,7 @@ export function IssuesList({
           <span className="hidden sm:inline">New Issue</span>
         </Button>
 
-        <div className="flex items-center gap-0.5 sm:gap-1">
+        <div className="flex items-center gap-0.5 sm:gap-1 shrink-0">
           {/* Filter */}
           <Popover>
             <PopoverTrigger asChild>