feat: render assistant replies as markdown

Add react-markdown + remark-math + rehype-katex to render the AI's
markdown (headings, bold, lists, code, blockquotes, tables) and LaTeX
math ($H_2O$ etc.). ReactMarkdown escapes raw HTML, keeping the stored
message rendering safe from XSS. Styled via .markdown-body in Message.css.
Applied to assistant messages only; user text stays plain.

frontend/package.json

@@ -9,8 +9,12 @@
     "preview": "vite preview"
   },
   "dependencies": {
+    "katex": "^0.18.4",
     "react": "^19.0.0",
-    "react-dom": "^19.0.0"
+    "react-dom": "^19.0.0",
+    "react-markdown": "^10.1.0",
+    "rehype-katex": "^7.0.1",
+    "remark-math": "^6.0.0"
   },
   "devDependencies": {
     "@vitejs/plugin-react": "^4.3.4",

frontend/src/components/message/Message.css

@@ -89,6 +89,88 @@
   color: var(--color-on-surface);
 }
 
+/* --- MARKDOWN RENDERED CONTENT (assistant) --- */
+.message__text.markdown-body {
+  white-space: normal;
+}
+.markdown-body > *:first-child { margin-top: 0; }
+.markdown-body > *:last-child { margin-bottom: 0; }
+.markdown-body p { margin: 0 0 12px; }
+.markdown-body h1,
+.markdown-body h2,
+.markdown-body h3,
+.markdown-body h4 {
+  margin: 20px 0 10px;
+  font-family: var(--font-heading);
+  font-weight: 700;
+  line-height: 1.3;
+}
+.markdown-body h1 { font-size: 22px; }
+.markdown-body h2 { font-size: 19px; }
+.markdown-body h3 { font-size: 16px; }
+.markdown-body h4 { font-size: 15px; }
+.markdown-body ul,
+.markdown-body ol {
+  margin: 0 0 12px;
+  padding-left: 22px;
+}
+.markdown-body li { margin-bottom: 4px; }
+.markdown-body blockquote {
+  margin: 0 0 12px;
+  padding: 6px 14px;
+  border-left: 4px solid var(--color-primary);
+  background: var(--color-surface-lowest);
+  border-radius: 0 8px 8px 0;
+  color: var(--color-on-surface-variant);
+}
+.markdown-body code {
+  font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
+  font-size: 0.9em;
+  background: rgba(0, 0, 0, 0.06);
+  padding: 1px 5px;
+  border-radius: 4px;
+}
+.markdown-body pre {
+  margin: 0 0 12px;
+  padding: 12px 14px;
+  background: #111827;
+  color: #e5e7eb;
+  border-radius: 8px;
+  overflow-x: auto;
+  white-space: pre;
+}
+.markdown-body pre code {
+  background: transparent;
+  padding: 0;
+  color: inherit;
+}
+.markdown-body hr {
+  margin: 18px 0;
+  border: none;
+  border-top: 1px solid rgba(0, 0, 0, 0.12);
+}
+.markdown-body table {
+  border-collapse: collapse;
+  margin: 0 0 12px;
+  width: 100%;
+}
+.markdown-body th,
+.markdown-body td {
+  border: 1px solid rgba(0, 0, 0, 0.12);
+  padding: 6px 10px;
+  text-align: left;
+}
+.markdown-body th {
+  background: rgba(0, 0, 0, 0.04);
+  font-weight: 600;
+}
+.markdown-body a {
+  color: var(--color-primary);
+  text-decoration: underline;
+}
+/* KaTeX math inherits font size */
+.markdown-body .katex { font-size: 1.05em; }
+
 /* --- ACTION BUTTONS --- */
 .message__actions {
   display: flex;

frontend/src/components/message/Message.jsx

@@ -3,6 +3,10 @@
    ======================================== */
 
 import { useEffect, useRef } from "react";
+import ReactMarkdown from "react-markdown";
+import remarkMath from "remark-math";
+import rehypeKatex from "rehype-katex";
+import "katex/dist/katex.min.css";
 import "./Message.css";
 
 const Message = ({ message }) => {
@@ -27,8 +31,19 @@ const Message = ({ message }) => {
         </div>
       )}
       <div className="message__bubble">
-        <div className="message__text">
+        <div className={`message__text ${!isUser ? "markdown-body" : ""}`}>
-          {text || (streaming ? "" : null)}
+          {isUser ? (
+            text
+          ) : (
+            text && (
+              <ReactMarkdown
+                remarkPlugins={[remarkMath]}
+                rehypePlugins={[rehypeKatex]}
+              >
+                {text}
+              </ReactMarkdown>
+            )
+          )}
           {streaming && <span className="message__typing-dots">
             <span className="message__typing-dot"></span>
             <span className="message__typing-dot"></span>