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/markdown_codefence_placeholder_regression.mjs

@@ -16,6 +16,10 @@ src = src.replace(
   /import \{ splitTableRow \} from '\.\/markdown\/tableRow\.js';/,
   'const splitTableRow = (row) => row.split("|").filter((cell) => cell.trim() !== "");'
 );
+src = src.replace(
+  /import \{ replaceEmojiShortcodes, hasEmojiShortcode \} from '\.\/emojiShortcodes\.js';/,
+  'const hasEmojiShortcode = (t) => !!t && t.indexOf(":") !== -1 && /:[a-z0-9_+-]{1,40}:/i.test(t); const replaceEmojiShortcodes = (t) => t;'
+);
 src = src.replace(/export function /g, 'function ');
 src = src.replace(/export const /g, 'const ');
 src = src.replace(/export default markdownModule;?/g, '');