fix(export): apply block quote syntax to every line in annotation export, closes #3534 (#3536)

Add formatBlockQuote utility and blockquote nunjucks filter so both
default and custom template exports prefix every line with `>`.
Document all available formatters in the custom template help panel.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/readest-app/src/utils/note.ts

@@ -213,6 +213,22 @@ env.addFilter('nl2br', (value: string) => {
   return value.replace(/\n/g, '<br>\n');
 });
 
+/**
+ * Formats text as a markdown block quote, prefixing every line with `> `.
+ */
+export function formatBlockQuote(text: string): string {
+  return text
+    .split('\n')
+    .map((line) => `> ${line}`)
+    .join('\n');
+}
+
+// Add 'blockquote' filter (prefix every line with > for markdown block quotes)
+env.addFilter('blockquote', (value: string) => {
+  if (!value || typeof value !== 'string') return '';
+  return formatBlockQuote(value);
+});
+
 /**
  * Renders a Jinja2/Nunjucks template with the given data
  * @param template The template string in Jinja2/Nunjucks syntax