feat(ui): UI/UX polish — layout, sidebar, button, theme improvements

- Fix global scrollbar overflow by removing h-svh from html element
- Add h-full overflow-hidden to html/body for proper app-like layout
- Fix default button variant: add shadow-sm and hover:bg-primary/90
- Update sidebar create-issue button to bg-background with shadow
- Add WorkspaceAvatar component and search/new-issue actions to sidebar header
- Improve theme provider with TooltipProvider wrapper
- Polish various page layouts, pickers, modals, and code block styling
- Clean up custom.css unused styles

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

apps/web/features/workspace/components/workspace-avatar.tsx

@@ -0,0 +1,29 @@
+import { cn } from "@/lib/utils";
+
+const sizeMap = {
+  sm: "h-5 w-5 text-[10px] rounded",
+  md: "h-7 w-7 text-xs rounded-md",
+  lg: "h-9 w-9 text-sm rounded-md",
+} as const;
+
+interface WorkspaceAvatarProps {
+  name: string;
+  size?: keyof typeof sizeMap;
+  className?: string;
+}
+
+function WorkspaceAvatar({ name, size = "sm", className }: WorkspaceAvatarProps) {
+  return (
+    <span
+      className={cn(
+        "inline-flex shrink-0 items-center justify-center border bg-muted font-semibold text-muted-foreground",
+        sizeMap[size],
+        className
+      )}
+    >
+      {name.charAt(0).toUpperCase()}
+    </span>
+  );
+}
+
+export { WorkspaceAvatar, type WorkspaceAvatarProps };