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/reasoning.tsx

@@ -7,10 +7,6 @@ import {
   CollapsibleTrigger,
 } from "@/components/ui/collapsible";
 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 { BrainIcon, ChevronDownIcon } from "lucide-react";
 import type { ComponentProps, ReactNode } from "react";
 import {
@@ -23,8 +19,8 @@ import {
   useRef,
   useState,
 } from "react";
-import { Streamdown } from "streamdown";
 
+import { Markdown } from "./markdown";
 import { Shimmer } from "./shimmer";
 
 interface ReasoningContextValue {
@@ -204,21 +200,23 @@ export type ReasoningContentProps = ComponentProps<
   children: string;
 };
 
-const streamdownPlugins = { cjk, code, math, mermaid };
-
 export const ReasoningContent = memo(
-  ({ className, children, ...props }: ReasoningContentProps) => (
-    <CollapsibleContent
-      className={cn(
-        "mt-4 text-sm",
-        "data-[state=closed]:fade-out-0 data-[state=closed]:slide-out-to-top-2 data-[state=open]:slide-in-from-top-2 text-muted-foreground outline-none data-[state=closed]:animate-out data-[state=open]:animate-in",
-        className
-      )}
-      {...props}
-    >
-      <Streamdown plugins={streamdownPlugins}>{children}</Streamdown>
-    </CollapsibleContent>
-  )
+  ({ className, children, ...props }: ReasoningContentProps) => {
+    const { isStreaming } = useReasoning();
+
+    return (
+      <CollapsibleContent
+        className={cn(
+          "mt-4 text-sm",
+          "data-[state=closed]:fade-out-0 data-[state=closed]:slide-out-to-top-2 data-[state=open]:slide-in-from-top-2 text-muted-foreground outline-none data-[state=closed]:animate-out data-[state=open]:animate-in",
+          className
+        )}
+        {...props}
+      >
+        <Markdown isAnimating={isStreaming}>{children}</Markdown>
+      </CollapsibleContent>
+    );
+  }
 );
 
 Reasoning.displayName = "Reasoning";