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/shopify/search-dialog.tsx

@@ -11,7 +11,7 @@ import {
   CommandItem,
 } from '@/components/ui/command';
 import { Button } from '@/components/ui/button';
-import { Loader } from '@/app/components/ui/loader';
+import { Loader } from '@/components/ui/loader';
 import { RiSearchLine, RiImageLine, RiCloseLine } from '@remixicon/react';
 import {
   searchSuggestions,
@@ -111,7 +111,7 @@ const SearchDialog: React.FC = () => {
               if (event.key === 'Enter') goToSearchPage();
             }}
           />
-          <div className="absolute right-2 top-0 flex h-9 items-center gap-1">
+          <div className="absolute right-2 top-0 flex h-12 items-center gap-1">
             {hasQuery && (
               <Button
                 onClick={() => setTerm('')}