fix: unescape markdown-escaped characters in math formulas to fix #608 (#637)

When editing reports, tiptap-markdown escapes special characters (*, _, [, ])
which corrupts LaTeX formulas. This fix:

1. Adds unescapeLatexInMath() function to reverse markdown escaping within
   math delimiters ($...$ and 94410...94410)
2. Applies the unescape function in the editor's onChange callback to clean
   the markdown before storing it
3. Adds comprehensive tests covering edge cases and round-trip scenarios

The fix ensures formulas like $(f * g)[n]$ remain unescaped when editing,
preventing display errors after save/reload.

web/src/core/utils/markdown.ts

@@ -2,6 +2,42 @@ export function autoFixMarkdown(markdown: string): string {
   return autoCloseTrailingLink(markdown);
 }
 
+/**
+ * Unescape markdown-escaped characters within math delimiters
+ * tiptap-markdown escapes special characters like *, _, [, ] which corrupts math formulas
+ * This function restores the original LaTeX by unescaping within $...$ and $$...$$
+ */
+export function unescapeLatexInMath(markdown: string): string {
+  let result = markdown;
+
+  // Process inline math: $...$
+  result = result.replace(/\$([^\$]+?)\$/g, (match, mathContent) => {
+    const unescaped = unescapeMarkdownSpecialChars(mathContent);
+    return `$${unescaped}$`;
+  });
+
+  // Process display math: $$...$$
+  result = result.replace(/\$\$([\s\S]+?)\$\$/g, (match, mathContent) => {
+    const unescaped = unescapeMarkdownSpecialChars(mathContent);
+    return `$$${unescaped}$$`;
+  });
+
+  return result;
+}
+
+/**
+ * Reverse markdown escaping for special characters
+ * Order matters: process \\ last to avoid re-escaping
+ */
+function unescapeMarkdownSpecialChars(text: string): string {
+  return text
+    .replace(/\\\*/g, '*')      // \* → *
+    .replace(/\\_/g, '_')       // \_ → _
+    .replace(/\\\[/g, '[')      // \[ → [
+    .replace(/\\\]/g, ']')      // \] → ]
+    .replace(/\\\\/g, '\\');    // \\ → \
+}
+
 /**
  * Normalize math delimiters for editor consumption
  * Converts display delimiters (\[...\], \\[...\\]) to $$ format