fix(#45): emoji autocomplete substring matching + check aliases

Match the typed text anywhere in the shortcode name (not just the prefix),
ranked exact > prefix > substring, so `:check` surfaces the checkmarks (✅ is
`white_check_mark`). Add check/checkmark/tick aliases for ✅ in a separate,
regen-safe map the chat + channel composers consume.

Closes #45

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/helpers/emoji_shortcodes.dart

@@ -1916,3 +1916,17 @@ const Map<String, String> emojiShortcodes = {
   'zombie_woman': '🧟‍♀️',
   'zzz': '💤',
 };
+
+// Hand-authored aliases (not gemoji), kept separate so regenerating the map
+// above never clobbers them. ✅ is `white_check_mark`, hence `check`.
+const Map<String, String> _emojiAliases = {
+  'check': '✅',
+  'checkmark': '✅',
+  'tick': '✅',
+};
+
+/// gemoji shortcodes plus [_emojiAliases] — what composers use.
+final Map<String, String> emojiShortcodesWithAliases = {
+  ...emojiShortcodes,
+  ..._emojiAliases,
+};

lib/screens/channel_chat_screen.dart

@@ -1250,7 +1250,7 @@ class _ChannelChatScreenState extends State<ChannelChatScreen> {
                       controller: _textController,
                       focusNode: _textFieldFocusNode,
                       candidates: _buildMentionCandidates(connector),
-                      emojiShortcodes: emojiShortcodes,
+                      emojiShortcodes: emojiShortcodesWithAliases,
                       hintText: context.l10n.chat_typeMessage,
                       onSubmitted: (_) => _sendMessage(),
                       encoder:

lib/screens/chat_screen.dart

@@ -627,7 +627,7 @@ class _ChatScreenState extends State<ChatScreen> {
                     controller: _textController,
                     focusNode: _textFieldFocusNode,
                     candidates: const [],
-                    emojiShortcodes: emojiShortcodes,
+                    emojiShortcodes: emojiShortcodesWithAliases,
                     hintText: context.l10n.chat_typeMessage,
                     onSubmitted: (_) => _sendMessage(connector),
                     encoder:

lib/widgets/mention_autocomplete.dart

@@ -277,14 +277,18 @@ class _MentionAutocompleteFieldState extends State<MentionAutocompleteField> {
         return;
       }
     }
-    final names = shortcodes.keys.where((k) => k.startsWith(query)).toList();
+    final names = shortcodes.keys.where((k) => k.contains(query)).toList();
     if (names.isEmpty) {
       _close();
       return;
     }
     names.sort((a, b) {
+      // Rank exact, then prefix, then mid-name matches; shorter names first.
       if (a == query && b != query) return -1;
       if (b == query && a != query) return 1;
+      final aPrefix = a.startsWith(query);
+      final bPrefix = b.startsWith(query);
+      if (aPrefix != bPrefix) return aPrefix ? -1 : 1;
       final byLen = a.length.compareTo(b.length);
       if (byLen != 0) return byLen;
       return a.compareTo(b);