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/icon/Icon.tsx

@@ -0,0 +1,57 @@
+import React from "react"
+import { cn } from "@/lib/utils"
+import { iconSpriteData } from "./sprite"
+import type { IconName } from "./icons"
+
+const SPRITE_ID = "openchamber-icon-sprite"
+
+let spriteInjected = false
+
+function ensureSpriteOnce() {
+  if (spriteInjected) return
+  if (typeof document === "undefined") return
+  const body = document.body
+  if (!body) return
+
+  const existing = document.getElementById(SPRITE_ID)
+  if (existing) {
+    spriteInjected = true
+    return
+  }
+
+  const svg = document.createElementNS("http://www.w3.org/2000/svg", "svg")
+  svg.id = SPRITE_ID
+  svg.setAttribute("aria-hidden", "true")
+  svg.style.display = "none"
+  svg.innerHTML = Object.entries(iconSpriteData)
+    .map(([name, content]) => `<symbol id="oc-${name}" viewBox="0 0 24 24">${content}</symbol>`)
+    .join("")
+  body.insertBefore(svg, body.firstChild)
+  spriteInjected = true
+}
+
+export interface IconProps extends React.ComponentPropsWithoutRef<"svg"> {
+  name: IconName
+}
+
+export const Icon = React.memo(({ name, className, ...rest }: IconProps) => {
+  // Inline sprite injection during render – must run before <use> tries
+  // to resolve the #oc-* reference during the same commit.
+  if (typeof document !== "undefined") {
+    ensureSpriteOnce()
+  }
+
+  return (
+    <svg
+      className={cn("remixicon", className)}
+      viewBox="0 0 24 24"
+      xmlns="http://www.w3.org/2000/svg"
+      aria-hidden="true"
+      {...rest}
+    >
+      <use href={`#oc-${name}`} />
+    </svg>
+  )
+})
+
+Icon.displayName = "Icon"