feat: add an accept button in view invitations page

frontend/lib/screens/pages/invitation_details_page.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import '../../models/invitation_models.dart';
 import '../../services/invitations_service.dart';
+import '../../services/user_service.dart';
 import '../../utils/invitation_utils.dart';
 
 class InvitationDetailsPage extends StatefulWidget {
@@ -23,11 +24,14 @@ class _InvitationDetailsPageState extends State<InvitationDetailsPage> {
   InvitationDetails? _invitationDetails;
   bool _isLoading = true;
   bool _isCancelling = false;
+  bool _isAccepting = false;
+  bool _isCurrentlyParticipant = false;
   String? _errorMessage;
 
   @override
   void initState() {
     super.initState();
+    _isCurrentlyParticipant = widget.isParticipant;
     _loadInvitationDetails();
   }
 
@@ -48,6 +52,11 @@ class _InvitationDetailsPageState extends State<InvitationDetailsPage> {
           _errorMessage = result['message'];
         }
       });
+      
+      // Update participation status after loading details
+      if (result['success']) {
+        await _updateParticipationStatus();
+      }
     }
   }
 
@@ -150,6 +159,52 @@ class _InvitationDetailsPageState extends State<InvitationDetailsPage> {
     }
   }
 
+  Future<void> _updateParticipationStatus() async {
+    if (_invitationDetails == null) return;
+    
+    final userResult = await UserService.getCurrentUser();
+    if (userResult['success'] && userResult['data'] != null) {
+      final currentUserId = userResult['data']['id'];
+      final isParticipant = _invitationDetails!.attendees.any((attendee) => attendee.id == currentUserId);
+      setState(() {
+        _isCurrentlyParticipant = isParticipant;
+      });
+    }
+  }
+
+  Future<void> _acceptInvitation() async {
+    setState(() {
+      _isAccepting = true;
+    });
+
+    final result = await InvitationsService.acceptInvitation(widget.invitationId);
+
+    if (mounted) {
+      setState(() {
+        _isAccepting = false;
+      });
+
+      if (result['success']) {
+        // Reload invitation details to reflect the new state
+        await _loadInvitationDetails();
+        
+        ScaffoldMessenger.of(context).showSnackBar(
+          SnackBar(
+            content: Text('Invitation accepted successfully!'),
+            backgroundColor: Colors.green,
+          ),
+        );
+      } else {
+        ScaffoldMessenger.of(context).showSnackBar(
+          SnackBar(
+            content: Text(result['message'] ?? 'Failed to accept invitation'),
+            backgroundColor: Colors.red,
+          ),
+        );
+      }
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
     return PopScope(
@@ -509,7 +564,45 @@ class _InvitationDetailsPageState extends State<InvitationDetailsPage> {
                           ),
                         ),
                       ],
-                      if (widget.isParticipant) ...[
+                      
+                      // Accept button for non-participants who are not owners
+                      if (!_isCurrentlyParticipant && !widget.isOwner) ...[
+                        SizedBox(height: 32),
+                        Container(
+                          width: double.infinity,
+                          height: 56,
+                          child: ElevatedButton(
+                            onPressed: _isAccepting ? null : _acceptInvitation,
+                            style: ElevatedButton.styleFrom(
+                              backgroundColor: Color(0xFF6A4C93),
+                              foregroundColor: Colors.white,
+                              shape: RoundedRectangleBorder(
+                                borderRadius: BorderRadius.circular(12),
+                              ),
+                              elevation: 2,
+                            ),
+                            child: _isAccepting
+                                ? SizedBox(
+                                    height: 20,
+                                    width: 20,
+                                    child: CircularProgressIndicator(
+                                      strokeWidth: 2,
+                                      valueColor: AlwaysStoppedAnimation<Color>(Colors.white),
+                                    ),
+                                  )
+                                : Text(
+                                    'Accept Invitation',
+                                    style: TextStyle(
+                                      fontSize: 16,
+                                      fontWeight: FontWeight.w600,
+                                    ),
+                                  ),
+                          ),
+                        ),
+                      ],
+                      
+                      // Cancel/Leave button for participants
+                      if (_isCurrentlyParticipant) ...[
                         SizedBox(height: 32),
                         Container(
                           width: double.infinity,

frontend/lib/screens/pages/invitations_page.dart

@@ -49,7 +49,9 @@ class _InvitationsPageState extends State<InvitationsPage>
                 _isLoading = false;
                 _errorMessage = null;
               });
-              print('Invitations UI updated with ${updatedInvitationsData.created.length + updatedInvitationsData.accepted.length + updatedInvitationsData.available.length} total invitations');
+              print(
+                'Invitations UI updated with ${updatedInvitationsData.created.length + updatedInvitationsData.accepted.length + updatedInvitationsData.available.length} total invitations',
+              );
             }
           },
           onError: (error) {
@@ -69,7 +71,9 @@ class _InvitationsPageState extends State<InvitationsPage>
       });
     }
 
-    final result = await InvitationsService.getAllInvitations(forceRefresh: forceRefresh);
+    final result = await InvitationsService.getAllInvitations(
+      forceRefresh: forceRefresh,
+    );
 
     if (mounted) {
       setState(() {
@@ -551,7 +555,10 @@ class _InvitationsPageState extends State<InvitationsPage>
           ),
           automaticallyImplyLeading: false,
           actions: [
-          IconButton(icon: Icon(Icons.refresh), onPressed: () => _loadInvitations(forceRefresh: true)),
+            IconButton(
+              icon: Icon(Icons.refresh),
+              onPressed: () => _loadInvitations(forceRefresh: true),
+            ),
           ],
         ),
         body: _isLoading