feat(todo): quick add suggests projects and labels as # and @ are typed (ADR 0033, D15)

Up to five projects or labels whose name, or a word of it, starts with
what follows the symbol show below the title; one tapped completes it.
None for a symbol inside a word (C#), nor once a name is typed whole.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/todo/halkyon_todo_flutter/lib/src/feature/task/widget/quick_add_sheet.dart

@@ -33,7 +33,9 @@ import 'package:material_ui/material_ui.dart';
 /// `@label`, `{deadline}`, `!!1`, `p1`, `!30m`, dates, times and
 /// recurrences in the app's language and English, each highlighted and
 /// shown on its chip; a tap on one keeps it as text, and a chip picked
-/// takes its token out of the title.
+/// takes its token out of the title. As `#` or `@` is typed, the projects
+/// or labels it may mean are offered below the title, one tapped
+/// completing it.
 Future<void> showQuickAdd(
   BuildContext context, {
   String? projectId,
@@ -55,6 +57,9 @@ Future<void> showQuickAdd(
 /// The key of quick add's title field.
 const quickAddTitleKey = ValueKey('quick-add-title');
 
+/// The key of the projects or labels suggested as `#` or `@` is typed.
+const quickAddSuggestionsKey = ValueKey('quick-add-suggestions');
+
 class const _QuickAdd({
   required final String? projectId,
   required final String? sectionId,
@@ -117,23 +122,95 @@ class _QuickAddState() extends State<_QuickAdd> {
       words: QuickAddWords.forLanguage(
         Localizations.localeOf(context).languageCode,
       ),
-      projects: [
-        for (final project in state.projects)
-          if (project.inbox) ...[
-            (id: project.id, name: l10n.shellTabInbox),
-            if (l10n.shellTabInbox != 'Inbox') (id: project.id, name: 'Inbox'),
-          ] else if (project.content case final content?)
-            (id: project.id, name: content.name),
-      ],
-      labels: [
-        for (final label in state.labels)
-          if (label.content case final content?)
-            (id: label.id, name: content.name),
-      ],
+      projects: _projects(l10n, state),
+      labels: _labels(state),
       ignored: _ignored,
     );
   }
 
+  /// The projects by the names `#` takes, the Inbox by its name in the
+  /// app's language and in English.
+  static List<NamedId> _projects(AppLocalizations l10n, TodoState state) => [
+    for (final project in state.projects)
+      if (project.inbox) ...[
+        (id: project.id, name: l10n.shellTabInbox),
+        if (l10n.shellTabInbox != 'Inbox') (id: project.id, name: 'Inbox'),
+      ] else if (project.content case final content?)
+        (id: project.id, name: content.name),
+  ];
+
+  static List<NamedId> _labels(TodoState state) => [
+    for (final label in state.labels)
+      if (label.content case final content?) (id: label.id, name: content.name),
+  ];
+
+  /// The `#` or `@` being typed just before the cursor: the symbol, where
+  /// it is, and what follows it; none where it stands inside a word.
+  ({String symbol, int start, String typed})? _typing() {
+    final selection = _title.selection;
+    if (!selection.isValid || !selection.isCollapsed) {
+      return null;
+    }
+    final text = _title.text;
+    final at = selection.baseOffset;
+    for (var i = at - 1; i >= 0 && at - i <= 40; i--) {
+      final char = text[i];
+      if (char == '#' || char == '@') {
+        if (i > 0 && text[i - 1].trim().isNotEmpty) {
+          return null;
+        }
+        return (symbol: char, start: i, typed: text.substring(i + 1, at));
+      }
+    }
+    return null;
+  }
+
+  /// The projects or labels whose name, or a word of it, starts with what
+  /// follows the `#` or `@` being typed, at most five; none once it names
+  /// one whole.
+  List<NamedId> _suggestions(AppLocalizations l10n, TodoState state) {
+    final typing = _typing();
+    if (typing == null) {
+      return const [];
+    }
+    final typed = typing.typed.toLowerCase();
+    final named = typing.symbol == '#'
+        ? _projects(l10n, state)
+        : _labels(state);
+    if (named.any((item) => item.name.toLowerCase() == typed)) {
+      return const [];
+    }
+    final seen = <String>{};
+    return [
+      for (final item in named)
+        if ((item.name.toLowerCase().startsWith(typed) ||
+                item.name
+                    .toLowerCase()
+                    .split(' ')
+                    .any((word) => word.startsWith(typed))) &&
+            seen.add(item.id))
+          item,
+    ].take(5).toList();
+  }
+
+  /// Completes the `#` or `@` being typed with [item]'s name.
+  void _complete(NamedId item) {
+    final typing = _typing();
+    if (typing == null) {
+      return;
+    }
+    final completed = '${typing.symbol}${item.name} ';
+    final end = typing.start + 1 + typing.typed.length;
+    final text = _title.text.replaceRange(typing.start, end, completed);
+    _title.value = TextEditingValue(
+      text: text,
+      selection: TextSelection.collapsed(
+        offset: typing.start + completed.length,
+      ),
+    );
+    _focus.requestFocus();
+  }
+
   /// Takes the tokens of [kinds] out of the title, a chip having been
   /// picked for them.
   void _dropTokens(Set<QuickAddKind> kinds) {
@@ -293,6 +370,7 @@ class _QuickAddState() extends State<_QuickAdd> {
       null => null,
     };
     final typing = _title.text.trim().isNotEmpty;
+    final suggestions = _suggestions(l10n, state);
     return SheetScaffold(
       title: widget.parentId == null ? l10n.taskAddTitle : l10n.subTaskAdd,
       hasSnackBars: true,
@@ -322,6 +400,29 @@ class _QuickAddState() extends State<_QuickAdd> {
               onSubmitted: (_) => _add(),
             ),
           ),
+          if (suggestions.isNotEmpty)
+            Material(
+              key: quickAddSuggestionsKey,
+              color: theme.colorScheme.surfaceContainerHigh,
+              borderRadius: BorderRadius.circular(12),
+              clipBehavior: Clip.antiAlias,
+              child: Column(
+                mainAxisSize: MainAxisSize.min,
+                children: [
+                  for (final item in suggestions)
+                    ListTile(
+                      dense: true,
+                      leading: Icon(switch (_typing()?.symbol) {
+                        '@' => LucideIcons.tag,
+                        _ when item.id == state.inboxId => LucideIcons.inbox,
+                        _ => LucideIcons.hash,
+                      }),
+                      title: Text(item.name),
+                      onTap: () => _complete(item),
+                    ),
+                ],
+              ),
+            ),
           if (_describing)
             TextField(
               controller: _description,