chore: insert emojis as UTF-8 Unicode instead of shortname syntax

[Copilot]

Emoji insertion via the picker and autocomplete was using :shortname: format. Native emojis should be inserted as their UTF-8 Unicode characters; custom emojis continue to use :shortname:.

Changes

• Picker (MessageBox.tsx): Resolves the selected emoji name against emoji.list — inserts UTF-8 for native, :shortname: for custom/unknown
• Autocomplete (: trigger, ComposerPopupProvider.tsx): getValue returns the Unicode character for native emojis; prefix set to '' to prevent the trigger : from being double-prepended
• Typed completion (MessageBox.tsx): keyup handler auto-converts a fully typed :emoji: shortname to UTF-8 the moment the closing : is pressed — no-ops on custom emojis and unknown shortnames

// Picker — before
chat.emojiPicker.open(ref, (emoji) => chat.composer?.insertText(` :${emoji}: `));

// Picker — after
chat.emojiPicker.open(ref, (emojiName) => {
  const emojiEntry = emoji.list[`:${emojiName}:`];
  const isCustom = emojiEntry?.emojiPackage === 'emojiCustom';
  const text = isCustom ? `:${emojiName}:` : (emojiEntry?.unicode ?? `:${emojiName}:`);
  chat.composer?.insertText(` ${text} `);
});

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