perf(ui): migrate icons to SVG sprite system

Replace @remixicon/react with a shared Icon component that renders
via <use href> references to a single hidden SVG sprite. This reduces
DOM node count by replacing inline SVGs with lightweight references.

- Create Icon component with sprite injection (packages/ui/src/components/icon/)
- Migrate all 164 files from @remixicon/react to Icon component
- Auto-generate sprite data from remixicon bundle (scripts/generate-icon-sprite.mjs)
- Add bun run icons:generate to package.json
- Move @remixicon/react to devDependencies
- Add icon usage instructions to theme-system skill

packages/ui/src/components/session/sidebar/SidebarActivitySections.tsx

@@ -1,8 +1,8 @@
 import React from 'react';
-import { RiArrowDownSLine, RiArrowRightSLine } from '@remixicon/react';
 import { cn } from '@/lib/utils';
 import type { SessionNode } from './types';
 import { useI18n } from '@/lib/i18n';
+import { Icon } from "@/components/icon/Icon";
 
 type ActivityItem = {
   node: SessionNode;
@@ -77,7 +77,7 @@ export function SidebarActivitySections({ sections, renderSessionNode }: Props):
               aria-expanded={!isCollapsed}
             >
               <span className="inline-flex h-4 w-4 items-center justify-center text-muted-foreground">
-                {isCollapsed ? <RiArrowRightSLine className="h-3.5 w-3.5" /> : <RiArrowDownSLine className="h-3.5 w-3.5" />}
+                {isCollapsed ? <Icon name="arrow-right-s" className="h-3.5 w-3.5" /> : <Icon name="arrow-down-s" className="h-3.5 w-3.5" />}
               </span>
               <span className="text-[14px] font-normal text-foreground/95">{section.title}</span>
             </button>