feat: add message translation support

- Introduced translation functionality in chat screen, allowing users to translate messages before sending.
- Added MessageTranslationButton to the input bar for enabling/disabling translation.
- Implemented translation service to handle incoming and outgoing text translations using llama models.
- Enhanced message storage to include original and translated text, language codes, and translation status.
- Created UI components for displaying translated messages and managing translation options.
- Added translation model management, including downloading and storing models locally.
- Updated app settings to manage translation preferences and model selections.

lib/widgets/message_translation_button.dart

@@ -0,0 +1,187 @@
+import 'package:flutter/material.dart';
+
+import '../l10n/l10n.dart';
+import '../models/translation_support.dart';
+
+class MessageTranslationButton extends StatelessWidget {
+  final bool enabled;
+  final String? languageCode;
+  final VoidCallback onPressed;
+
+  const MessageTranslationButton({
+    super.key,
+    required this.enabled,
+    required this.languageCode,
+    required this.onPressed,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    final label = _languageLabel(
+      languageCode,
+      context.l10n.translation_systemLanguage,
+    );
+    return IconButton(
+      icon: Icon(enabled ? Icons.translate : Icons.translate_outlined),
+      onPressed: onPressed,
+      tooltip: enabled
+          ? context.l10n.translation_translateTo(label)
+          : context.l10n.translation_translationOptions,
+    );
+  }
+}
+
+Future<void> showMessageTranslationSheet({
+  required BuildContext context,
+  required bool enabled,
+  required String? selectedLanguageCode,
+  required ValueChanged<bool> onEnabledChanged,
+  required ValueChanged<String> onLanguageSelected,
+}) {
+  return showModalBottomSheet<void>(
+    context: context,
+    isScrollControlled: true,
+    builder: (context) => _MessageTranslationSheet(
+      enabled: enabled,
+      selectedLanguageCode: selectedLanguageCode,
+      onEnabledChanged: onEnabledChanged,
+      onLanguageSelected: onLanguageSelected,
+    ),
+  );
+}
+
+class _MessageTranslationSheet extends StatefulWidget {
+  final bool enabled;
+  final String? selectedLanguageCode;
+  final ValueChanged<bool> onEnabledChanged;
+  final ValueChanged<String> onLanguageSelected;
+
+  const _MessageTranslationSheet({
+    required this.enabled,
+    required this.selectedLanguageCode,
+    required this.onEnabledChanged,
+    required this.onLanguageSelected,
+  });
+
+  @override
+  State<_MessageTranslationSheet> createState() =>
+      _MessageTranslationSheetState();
+}
+
+class _MessageTranslationSheetState extends State<_MessageTranslationSheet> {
+  late final TextEditingController _searchController;
+  late bool _localEnabled;
+  late String? _localSelectedLanguageCode;
+  List<TranslationLanguageOption> _filtered = supportedTranslationLanguages;
+
+  @override
+  void initState() {
+    super.initState();
+    _searchController = TextEditingController();
+    _localEnabled = widget.enabled;
+    _localSelectedLanguageCode = widget.selectedLanguageCode;
+  }
+
+  @override
+  void dispose() {
+    _searchController.dispose();
+    super.dispose();
+  }
+
+  void _updateFilter(String query) {
+    final normalized = query.trim().toLowerCase();
+    setState(() {
+      _filtered = supportedTranslationLanguages.where((option) {
+        return option.label.toLowerCase().contains(normalized) ||
+            option.code.toLowerCase().contains(normalized);
+      }).toList();
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return SafeArea(
+      child: Padding(
+        padding: EdgeInsets.only(
+          left: 16,
+          right: 16,
+          top: 16,
+          bottom: 16 + MediaQuery.of(context).viewInsets.bottom,
+        ),
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          crossAxisAlignment: CrossAxisAlignment.start,
+          children: [
+            Text(
+              context.l10n.translation_messageTranslation,
+              style: const TextStyle(fontSize: 18, fontWeight: FontWeight.bold),
+            ),
+            const SizedBox(height: 8),
+            SwitchListTile(
+              contentPadding: EdgeInsets.zero,
+              title: Text(context.l10n.translation_translateBeforeSending),
+              subtitle: Text(
+                _localEnabled
+                    ? context.l10n.translation_composerEnabledHint
+                    : context.l10n.translation_composerDisabledHint,
+              ),
+              value: _localEnabled,
+              onChanged: (value) {
+                setState(() => _localEnabled = value);
+                widget.onEnabledChanged(value);
+              },
+            ),
+            const SizedBox(height: 8),
+            TextField(
+              controller: _searchController,
+              onChanged: _updateFilter,
+              decoration: InputDecoration(
+                labelText: context.l10n.translation_targetLanguage,
+                prefixIcon: const Icon(Icons.search),
+                border: const OutlineInputBorder(),
+              ),
+            ),
+            const SizedBox(height: 12),
+            Flexible(
+              child: ListView.builder(
+                shrinkWrap: true,
+                itemCount: _filtered.length,
+                itemBuilder: (context, index) {
+                  final option = _filtered[index];
+                  final selected = option.code == _localSelectedLanguageCode;
+                  return ListTile(
+                    contentPadding: EdgeInsets.zero,
+                    leading: Icon(
+                      selected
+                          ? Icons.radio_button_checked
+                          : Icons.radio_button_unchecked,
+                    ),
+                    title: Text(option.label),
+                    subtitle: Text(option.code.toUpperCase()),
+                    onTap: () {
+                      setState(() => _localSelectedLanguageCode = option.code);
+                      widget.onLanguageSelected(option.code);
+                      Navigator.pop(context);
+                    },
+                  );
+                },
+              ),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}
+
+String _languageLabel(String? languageCode, String systemLanguageFallback) {
+  if (languageCode == null) {
+    return systemLanguageFallback;
+  }
+  for (final option in supportedTranslationLanguages) {
+    if (option.code == languageCode) {
+      return option.label;
+    }
+  }
+  return languageCode.toUpperCase();
+}

lib/widgets/translated_message_content.dart

@@ -0,0 +1,61 @@
+import 'package:flutter/material.dart';
+
+import '../helpers/link_handler.dart';
+
+class TranslatedMessageContent extends StatelessWidget {
+  final String displayText;
+  final String? originalText;
+  final TextStyle style;
+  final TextStyle? originalStyle;
+  final bool showOriginalFirst;
+
+  const TranslatedMessageContent({
+    super.key,
+    required this.displayText,
+    required this.style,
+    this.originalText,
+    this.originalStyle,
+    this.showOriginalFirst = true,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    final trimmedDisplay = displayText.trim();
+    final trimmedOriginal = originalText?.trim();
+    final shouldShowOriginal =
+        trimmedOriginal != null &&
+        trimmedOriginal.isNotEmpty &&
+        trimmedOriginal != trimmedDisplay;
+    final originalWidget = shouldShowOriginal
+        ? LinkHandler.buildLinkifyText(
+            context: context,
+            text: trimmedOriginal,
+            style:
+                originalStyle ??
+                style.copyWith(
+                  fontStyle: FontStyle.italic,
+                  fontSize: style.fontSize,
+                ),
+          )
+        : null;
+    final translatedWidget = LinkHandler.buildLinkifyText(
+      context: context,
+      text: trimmedDisplay,
+      style: style,
+    );
+
+    if (!shouldShowOriginal) {
+      return translatedWidget;
+    }
+
+    final children = showOriginalFirst
+        ? [originalWidget!, const SizedBox(height: 6), translatedWidget]
+        : [translatedWidget, const SizedBox(height: 6), originalWidget!];
+
+    return Column(
+      crossAxisAlignment: CrossAxisAlignment.start,
+      mainAxisSize: MainAxisSize.min,
+      children: children,
+    );
+  }
+}