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/chat/TurnChangedFilesDropdown.tsx

@@ -1,5 +1,4 @@
 import React from 'react';
-import { RiFileEditLine, RiArrowDownSLine, RiArrowUpSLine } from '@remixicon/react';
 import type { ToolPart } from '@opencode-ai/sdk/v2';
 import { Popover } from '@base-ui/react/popover';
 import { useDirectoryStore } from '@/stores/useDirectoryStore';
@@ -17,6 +16,7 @@ import {
 import { ChangedFilesList } from './ChangedFilesList';
 import { changedFilesPopoverClassName, changedFilesPopoverStyle } from './changedFilesPopover';
 import { Tooltip, TooltipContent, TooltipTrigger } from '@/components/ui/tooltip';
+import { Icon } from "@/components/icon/Icon";
 import type { TurnActivityRecord } from './lib/turns/types';
 
 interface TurnChangedFilesDropdownProps {
@@ -96,12 +96,12 @@ export const TurnChangedFilesDropdown: React.FC<TurnChangedFilesDropdownProps> =
                                 onPointerDownCapture={syncPortalContainer}
                                 onFocusCapture={syncPortalContainer}
                             >
-                                <RiFileEditLine className="h-3.5 w-3.5" />
+                                <Icon name="file-edit" className="h-3.5 w-3.5" />
                                 <span className="message-footer__label">{label}</span>
                                 {isExpanded ? (
-                                    <RiArrowUpSLine className="h-3.5 w-3.5" />
+                                    <Icon name="arrow-up-s" className="h-3.5 w-3.5" />
                                 ) : (
-                                    <RiArrowDownSLine className="h-3.5 w-3.5" />
+                                    <Icon name="arrow-down-s" className="h-3.5 w-3.5" />
                                 )}
                             </button>
                         }