refactor: migrate Phase 1 providers (2-7/10) to @riverpod

Migrated providers:
- selectedModelProvider → SelectedModel
- isManualModelSelectionProvider → IsManualModelSelection
- reviewerModeProvider → ReviewerMode
- isLoadingConversationProvider → IsLoadingConversation
- prefilledInputTextProvider → PrefilledInputText
- inputFocusTriggerProvider → InputFocusTrigger
- composerHasFocusProvider → ComposerHasFocus
- batchModeProvider → BatchMode
- reducedMotionProvider → ReducedMotion

All provider names unchanged, no breaking changes.
Build runner successful, analyzer passing.
Only 1 WARNING (keepAlive usage) and 2 INFO items remaining.

lib/core/services/animation_service.dart

@@ -1,7 +1,11 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:riverpod_annotation/riverpod_annotation.dart';
+
 import '../../shared/theme/theme_extensions.dart';
 
+part 'animation_service.g.dart';
+
 /// Service for managing animations with performance optimization and accessibility
 class AnimationService {
   /// Get optimized animation duration based on context and settings
@@ -208,9 +212,13 @@ class AnimationService {
 enum PageTransitionType { fade, slide, scale }
 
 /// Provider for reduced motion preference
-final reducedMotionProvider = NotifierProvider<ReducedMotionNotifier, bool>(
-  ReducedMotionNotifier.new,
-);
+@riverpod
+class ReducedMotion extends _$ReducedMotion {
+  @override
+  bool build() => false;
+
+  void set(bool value) => state = value;
+}
 
 /// Provider for animation performance settings
 final animationPerformanceProvider =
@@ -218,13 +226,6 @@ final animationPerformanceProvider =
       AnimationPerformanceNotifier.new,
     );
 
-class ReducedMotionNotifier extends Notifier<bool> {
-  @override
-  bool build() => false;
-
-  void set(bool value) => state = value;
-}
-
 class AnimationPerformanceNotifier extends Notifier<AnimationPerformance> {
   @override
   AnimationPerformance build() => AnimationPerformance.adaptive;