Replace Streamdown with a plugin-free markdown renderer

The assistant only ever renders prose, links and the odd list, but
Streamdown's plugin set dragged in shiki, mermaid and katex — and
streamdown core declares mermaid as a hard dependency, so dropping the
plugins alone would have left it installed. Rendering now runs on the
same unified pipeline Streamdown used internally, minus the plugin
surface: remark-parse -> remark-gfm -> remark-rehype -> rehype-sanitize
-> hast-util-to-jsx-runtime. GFM, streaming repair and the link-safety
hook all survive; fenced code renders as plain <pre>. Net -264/+101
packages.

remend now runs only while a part is still streaming. On settled text it
reads `*$89*` as an unclosed italic — the `$` throws off its
closing-delimiter scan — and appends a stray `*` that parses as an empty
bullet. Italic prices are ordinary storefront copy, so that would have
shown up in production.

Route-relative links go through next/link in the same tab; sending a
shopper to a product page in a new tab would strand the conversation
behind it. Only external destinations get the confirmation dialog.

code-block.tsx and tool.tsx were the sole users of shiki and had no
importers, so both are gone.

Also folded in two unrelated changes that were already in the tree: the
loader move out of app/components/ui, and dropping the fade on the
command palette so it opens instantly.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KuySwKLSF4uEPpZjHkdHBP

components/ai-elements/message.tsx

@@ -12,23 +12,19 @@ import {
   TooltipTrigger,
 } from "@/components/ui/tooltip";
 import { cn } from "@/lib/utils";
-import { cjk } from "@streamdown/cjk";
-import { code } from "@streamdown/code";
-import { math } from "@streamdown/math";
-import { mermaid } from "@streamdown/mermaid";
 import type { UIMessage } from "ai";
 import { ChevronLeftIcon, ChevronRightIcon } from "lucide-react";
 import type { ComponentProps, HTMLAttributes, ReactElement } from "react";
 import {
   createContext,
-  memo,
   useCallback,
   useContext,
   useEffect,
   useMemo,
   useState,
 } from "react";
-import { Streamdown } from "streamdown";
+
+import { Markdown } from "./markdown";
 
 export type MessageProps = HTMLAttributes<HTMLDivElement> & {
   from: UIMessage["role"];
@@ -319,27 +315,10 @@ export const MessageBranchPage = ({
   );
 };
 
-export type MessageResponseProps = ComponentProps<typeof Streamdown>;
+export type MessageResponseProps = ComponentProps<typeof Markdown>;
 
-const streamdownPlugins = { cjk, code, math, mermaid };
-
-export const MessageResponse = memo(
-  ({ className, ...props }: MessageResponseProps) => (
-    <Streamdown
-      className={cn(
-        "size-full [&>*:first-child]:mt-0 [&>*:last-child]:mb-0",
-        className
-      )}
-      plugins={streamdownPlugins}
-      {...props}
-    />
-  ),
-  (prevProps, nextProps) =>
-    prevProps.children === nextProps.children &&
-    nextProps.isAnimating === prevProps.isAnimating
-);
-
-MessageResponse.displayName = "MessageResponse";
+// Markdown already memoises on children/isAnimating.
+export const MessageResponse = Markdown;
 
 export type MessageToolbarProps = ComponentProps<"div">;