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

package.json

@@ -16,16 +16,13 @@
         "@radix-ui/react-use-controllable-state": "^1.2.6",
         "@remixicon/react": "^4.9.0",
         "@shopify/storefront-api-client": "^1.0.0",
-        "@streamdown/cjk": "^1.0.3",
-        "@streamdown/code": "^1.1.1",
-        "@streamdown/math": "^1.0.2",
-        "@streamdown/mermaid": "^1.0.2",
         "ai": "^7",
         "class-variance-authority": "0.7.1",
         "clsx": "^2.1.1",
         "cmdk": "^1.1.1",
         "embla-carousel-react": "^8.6.0",
         "framer-motion": "12.42.2",
+        "hast-util-to-jsx-runtime": "^2.3.6",
         "input-otp": "^1.4.2",
         "lucide-react": "^0.562.0",
         "motion": "^12.23.26",
@@ -34,18 +31,23 @@
         "radix-ui": "^1.6.7",
         "react": "^19.0.0",
         "react-dom": "^19.0.0",
-        "shiki": "^3.19.0",
+        "rehype-sanitize": "^6.0.0",
+        "remark-gfm": "^4.0.1",
+        "remark-parse": "^11.0.0",
+        "remark-rehype": "^11.1.2",
+        "remend": "^1.3.0",
         "sonner": "^2.0.7",
-        "streamdown": "^2.5.0",
         "tailwind-merge": "^3.4.0",
         "tailwindcss": "^4",
         "tw-animate-css": "1.4.0",
+        "unified": "^11.0.5",
         "use-stick-to-bottom": "^1.1.6",
         "zod": "^4.4.3",
         "zustand": "^5.0.11"
     },
     "devDependencies": {
         "@tailwindcss/postcss": "^4",
+        "@types/hast": "^3.0.5",
         "@types/node": "^22",
         "@types/react": "^19",
         "@types/react-dom": "^19",