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/ui/loader.tsx

@@ -0,0 +1,96 @@
+import { cn } from "@/lib/utils";
+import type { HTMLAttributes } from "react";
+
+interface LoaderIconProps {
+  size?: number;
+}
+
+const LoaderIcon = ({ size = 16 }: LoaderIconProps) => (
+  <svg
+    height={size}
+    strokeLinejoin="round"
+    style={{ color: "currentcolor" }}
+    viewBox="0 0 16 16"
+    width={size}
+  >
+    <title>Loader</title>
+    <g clipPath="url(#clip0_2393_1490)">
+      <path d="M8 0V4" stroke="currentColor" strokeWidth="1.5" />
+      <path
+        d="M8 16V12"
+        opacity="0.5"
+        stroke="currentColor"
+        strokeWidth="1.5"
+      />
+      <path
+        d="M3.29773 1.52783L5.64887 4.7639"
+        opacity="0.9"
+        stroke="currentColor"
+        strokeWidth="1.5"
+      />
+      <path
+        d="M12.7023 1.52783L10.3511 4.7639"
+        opacity="0.1"
+        stroke="currentColor"
+        strokeWidth="1.5"
+      />
+      <path
+        d="M12.7023 14.472L10.3511 11.236"
+        opacity="0.4"
+        stroke="currentColor"
+        strokeWidth="1.5"
+      />
+      <path
+        d="M3.29773 14.472L5.64887 11.236"
+        opacity="0.6"
+        stroke="currentColor"
+        strokeWidth="1.5"
+      />
+      <path
+        d="M15.6085 5.52783L11.8043 6.7639"
+        opacity="0.2"
+        stroke="currentColor"
+        strokeWidth="1.5"
+      />
+      <path
+        d="M0.391602 10.472L4.19583 9.23598"
+        opacity="0.7"
+        stroke="currentColor"
+        strokeWidth="1.5"
+      />
+      <path
+        d="M15.6085 10.4722L11.8043 9.2361"
+        opacity="0.3"
+        stroke="currentColor"
+        strokeWidth="1.5"
+      />
+      <path
+        d="M0.391602 5.52783L4.19583 6.7639"
+        opacity="0.8"
+        stroke="currentColor"
+        strokeWidth="1.5"
+      />
+    </g>
+    <defs>
+      <clipPath id="clip0_2393_1490">
+        <rect fill="white" height="16" width="16" />
+      </clipPath>
+    </defs>
+  </svg>
+);
+
+export type LoaderProps = HTMLAttributes<HTMLDivElement> & {
+  size?: number;
+};
+
+export const Loader = ({ className, size = 16, ...props }: LoaderProps) => (
+  <div
+    className={cn(
+      "inline-flex animate-spin items-center justify-center",
+      className
+    )}
+    {...props}
+  >
+    <LoaderIcon size={size} />
+  </div>
+);