Add markdown :name: emoji shortcode support in messages (#27)

Typing a complete 😂/😉/etc. shortcode now renders as the emoji,
matching Slack/GitHub/Discord. Render-time only, alongside the existing
preserveLineBreaks preprocessing step -- stored/sent content keeps the
raw :name: text, same as markdown itself is never converted until
display. Fenced code blocks and inline code spans are skipped so
pasted code (a Ruby symbol, a dict key) isn't silently mangled.

frontend/src/lib/emojiShortcodes.ts is generated once from
emojibase-data's GitHub shortcode set (same one-time-generator approach
as #19's emojiNames.ts, never a runtime dependency) -- 928 of the
app's 936 emoji matched, 956 aliases, no collisions.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/MessageContent.tsx

@@ -1,4 +1,5 @@
 import Markdown from 'markdown-to-jsx'
+import { EMOJI_SHORTCODES } from '../lib/emojiShortcodes'
 
 interface MessageContentProps {
   content: string
@@ -23,6 +24,38 @@ function MarkdownImageLink({ src, alt, title }: MarkdownImageLinkProps) {
   )
 }
 
+const SHORTCODE_PATTERN = /:([a-z0-9_+-]+):/g
+
+// Converts a complete `:name:` shortcode to its emoji, skipping fenced code
+// blocks and inline code spans -- someone pasting code containing
+// `:something:` (a Ruby symbol, a dict key) shouldn't get it silently
+// turned into an emoji. This is render-time only: stored/sent content
+// always keeps the literal `:name:` text, matching how markdown itself is
+// never converted until display.
+function convertShortcodes(text: string): string {
+  const lines = text.split('\n')
+  let inFence = false
+  return lines
+    .map((line) => {
+      if (/^\s*```/.test(line)) {
+        inFence = !inFence
+        return line
+      }
+      if (inFence) return line
+      // Splitting on backtick-delimited spans keeps inline code (`:foo:`)
+      // untouched -- odd-indexed segments are the code spans themselves.
+      return line
+        .split(/(`+[^`]*`+)/g)
+        .map((part, i) =>
+          i % 2 === 0
+            ? part.replace(SHORTCODE_PATTERN, (match, name) => EMOJI_SHORTCODES[name] ?? match)
+            : part,
+        )
+        .join('')
+    })
+    .join('\n')
+}
+
 // CommonMark treats a single newline as a soft break (rendered as a space),
 // not a visible line break -- only a trailing double-space or blank line
 // produces one. The Composer's Shift+Enter has always inserted a plain
@@ -60,5 +93,5 @@ export const MARKDOWN_OPTIONS = {
 }
 
 export function MessageContent({ content }: MessageContentProps) {
-  return <Markdown options={MARKDOWN_OPTIONS}>{preserveLineBreaks(content)}</Markdown>
+  return <Markdown options={MARKDOWN_OPTIONS}>{preserveLineBreaks(convertShortcodes(content))}</Markdown>
 }