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/store-assistant.tsx

@@ -393,6 +393,11 @@ const StoreAssistant: React.FC = () => {
                       return (
                         <MessageResponse
                           key={index}
+                          // Only repair half-written markdown while it streams;
+                          // settled text is rendered exactly as sent.
+                          isAnimating={
+                            status === 'streaming' && part.state === 'streaming'
+                          }
                           linkSafety={{
                             enabled: true,
                             onLinkCheck: isInternalLink,