feat: custom GLSL shader import

close #624

lib/providers/shader_provider.dart

@@ -2,6 +2,7 @@ import 'package:flutter/foundation.dart';
 
 import '../models/shader_preset.dart';
 import '../services/settings_service.dart';
+import '../services/shader_asset_loader.dart';
 
 /// Provider for managing shader preset state.
 ///
@@ -11,6 +12,7 @@ class ShaderProvider extends ChangeNotifier {
 
   ShaderPreset _savedPreset = ShaderPreset.none;
   ShaderPreset _currentPreset = ShaderPreset.none;
+  List<ShaderPreset> _customPresets = [];
   bool _initialized = false;
 
   ShaderProvider() {
@@ -20,8 +22,12 @@ class ShaderProvider extends ChangeNotifier {
   Future<void> _initialize() async {
     _settingsService = await SettingsService.getInstance();
 
+    // Load custom presets from storage
+    final customData = _settingsService.getCustomShaderPresets();
+    _customPresets = customData.map((json) => ShaderPreset.fromJson(json)).toList();
+
     final presetId = _settingsService.getGlobalShaderPreset();
-    _savedPreset = ShaderPreset.fromId(presetId) ?? ShaderPreset.none;
+    _savedPreset = findPresetById(presetId) ?? ShaderPreset.none;
     _currentPreset = _savedPreset;
 
     _initialized = true;
@@ -37,12 +43,20 @@ class ShaderProvider extends ChangeNotifier {
   /// The currently active shader preset
   ShaderPreset get currentPreset => _currentPreset;
 
-  /// All available shader presets
-  List<ShaderPreset> get allPresets => ShaderPreset.allPresets;
+  /// All available shader presets (built-in + custom)
+  List<ShaderPreset> get allPresets => [...ShaderPreset.allPresets, ..._customPresets];
+
+  /// Custom shader presets only
+  List<ShaderPreset> get customPresets => _customPresets;
 
   /// Whether any shader is currently enabled
   bool get isShaderEnabled => _currentPreset.type != ShaderPresetType.none;
 
+  /// Find a preset by its ID, searching both built-in and custom presets.
+  ShaderPreset? findPresetById(String id) {
+    return ShaderPreset.fromId(id) ?? _customPresets.cast<ShaderPreset?>().firstWhere((p) => p!.id == id, orElse: () => null);
+  }
+
   /// Apply and persist a shader preset
   Future<void> setPreset(ShaderPreset preset) async {
     _savedPreset = preset;
@@ -59,6 +73,48 @@ class ShaderProvider extends ChangeNotifier {
     }
   }
 
+  /// Import a custom shader from a file path.
+  /// Copies the file to the custom shaders directory and creates a preset.
+  Future<ShaderPreset> importCustomShader(String filePath, String displayName) async {
+    final storedFileName = await ShaderAssetLoader.importCustomShader(filePath);
+    final id = 'custom_$storedFileName';
+
+    final preset = ShaderPreset(
+      id: id,
+      name: displayName,
+      type: ShaderPresetType.custom,
+      fileName: storedFileName,
+    );
+
+    _customPresets.add(preset);
+    await _saveCustomPresets();
+    notifyListeners();
+    return preset;
+  }
+
+  /// Delete a custom shader preset and its file.
+  Future<void> deleteCustomShader(ShaderPreset preset) async {
+    if (preset.fileName != null) {
+      await ShaderAssetLoader.deleteCustomShader(preset.fileName!);
+    }
+    _customPresets.removeWhere((p) => p.id == preset.id);
+    await _saveCustomPresets();
+
+    // Reset to none if the deleted preset was active
+    if (_currentPreset.id == preset.id || _savedPreset.id == preset.id) {
+      _savedPreset = ShaderPreset.none;
+      _currentPreset = ShaderPreset.none;
+      await _settingsService.setGlobalShaderPreset(ShaderPreset.none.id);
+    }
+
+    notifyListeners();
+  }
+
+  Future<void> _saveCustomPresets() async {
+    final data = _customPresets.map((p) => p.toJson()).toList();
+    await _settingsService.setCustomShaderPresets(data);
+  }
+
   /// Reset to default (no shaders)
   Future<void> reset() async {
     await setPreset(ShaderPreset.none);