Render emoji shortcodes as icons in chat (#345) (#629)

Chat models often emit GitHub/Slack-style :shortcode: text (e.g. 😊,
🎤) instead of the actual emoji. The renderer only converted real
Unicode emoji to the monochrome line icons, so shortcodes rendered as literal
text.

Add a pure, browser-free shortcode->Unicode map (emojiShortcodes.js) and run it
inside svgifyEmoji ahead of the existing Unicode->SVG pass, skipping <code>/<pre>
so code stays literal. Covers ~430 common shortcodes plus common aliases
(+1/thumbsup, etc.).

Keep the conversion from touching anything it shouldn't:
* Scope it to chat. mdToHtml/svgifyEmoji take a { shortcodes } option (default
  on); document and email body rendering (compose, export, preview) pass it as
  false so author-typed :shortcode: text stays literal. The Unicode->SVG pass
  still runs there exactly as before.
* Only convert a :shortcode: that stands on its own. A word-boundary guard
  leaves embedded colon runs alone, so "1:100:2", "10:30:45", "16:9" and
  host:fire:port are never rewritten.

Tests: extend the node-driven unit test with the boundary/false-positive cases,
and fix the markdown-rendering test loader to resolve the new emojiShortcodes
import.

tests/test_markdown_rendering_js.py

@@ -41,6 +41,18 @@ def _run_markdown_case(markdown: str, render_expr: str = "mod.mdToHtml(input)"):
             return (row || '').replace(/^\\s*\\|/, '').replace(/\\|\\s*$/, '').split('|').map(c => c.trim());
           }`
         );
+        // markdown.js imports the emoji-shortcode helpers relatively (issue #345),
+        // which a data: URL module can't resolve. Inline the REAL helpers (minus
+        // their export keywords) so the renderer's shortcode pass behaves exactly
+        // as it does in the browser.
+        const emojiSource = fs.readFileSync('./static/js/emojiShortcodes.js', 'utf8')
+          .replace(/^export default .*$/m, '')
+          .replace(/export const /g, 'const ')
+          .replace(/export function /g, 'function ');
+        source = source.replace(
+          /import \{ replaceEmojiShortcodes, hasEmojiShortcode \} from ['"]\.\/emojiShortcodes\.js['"];/,
+          () => emojiSource
+        );
         source = source.replace(
           /var escapeHtml = uiModule\.esc;/,
           `var escapeHtml = (value) => String(value ?? '')