feat(activities): add autocomplete & what's new popup

Features:
- Add autocomplete support for Activity search with Google Places API.
- Add "What's New" popup system to showcase new features on app update.
- Implement logic to detect fresh installs vs updates.

Fixes:
- Switch API key handling to use Firebase config for Release mode support.
- Refactor map pins to be consistent (red pins).
- UI fixes on Create Trip page (overflow issues).

Refactor:
- Make WhatsNewDialog reusable by accepting features list as parameter.

lib/components/whats_new_dialog.dart

@@ -0,0 +1,140 @@
+import 'package:flutter/material.dart';
+import '../../services/whats_new_service.dart';
+
+class WhatsNewDialog extends StatelessWidget {
+  final VoidCallback onDismiss;
+  final List<WhatsNewItem> features;
+
+  const WhatsNewDialog({
+    super.key,
+    required this.onDismiss,
+    required this.features,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+
+    return Dialog(
+      shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(20)),
+      backgroundColor: theme.colorScheme.surface,
+      child: Padding(
+        padding: const EdgeInsets.all(24.0),
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          crossAxisAlignment: CrossAxisAlignment.start,
+          children: [
+            // Header
+            Row(
+              children: [
+                Container(
+                  padding: const EdgeInsets.all(8),
+                  decoration: BoxDecoration(
+                    color: theme.colorScheme.primaryContainer,
+                    shape: BoxShape.circle,
+                  ),
+                  child: Icon(
+                    Icons.auto_awesome,
+                    color: theme.colorScheme.primary,
+                    size: 24,
+                  ),
+                ),
+                const SizedBox(width: 16),
+                Expanded(
+                  child: Text(
+                    'Quoi de neuf ?',
+                    style: theme.textTheme.headlineSmall?.copyWith(
+                      fontWeight: FontWeight.bold,
+                    ),
+                  ),
+                ),
+              ],
+            ),
+            const SizedBox(height: 24),
+
+            // Features List
+            Flexible(
+              child: ListView.separated(
+                shrinkWrap: true,
+                itemCount: features.length,
+                separatorBuilder: (_, __) => const SizedBox(height: 16),
+                itemBuilder: (context, index) {
+                  final feature = features[index];
+                  return Row(
+                    crossAxisAlignment: CrossAxisAlignment.start,
+                    children: [
+                      Icon(
+                        feature.icon,
+                        color: theme.colorScheme.primary,
+                        size: 20,
+                      ),
+                      const SizedBox(width: 12),
+                      Expanded(
+                        child: Column(
+                          crossAxisAlignment: CrossAxisAlignment.start,
+                          children: [
+                            Text(
+                              feature.title,
+                              style: theme.textTheme.titleMedium?.copyWith(
+                                fontWeight: FontWeight.w600,
+                              ),
+                            ),
+                            const SizedBox(height: 4),
+                            Text(
+                              feature.description,
+                              style: theme.textTheme.bodyMedium?.copyWith(
+                                color: theme.colorScheme.onSurfaceVariant,
+                              ),
+                            ),
+                          ],
+                        ),
+                      ),
+                    ],
+                  );
+                },
+              ),
+            ),
+
+            const SizedBox(height: 32),
+
+            // Button
+            SizedBox(
+              width: double.infinity,
+              height: 50,
+              child: ElevatedButton(
+                onPressed: () {
+                  // Marquer comme vu via le service
+                  WhatsNewService().markCurrentVersionAsSeen();
+                  onDismiss();
+                },
+                style: ElevatedButton.styleFrom(
+                  backgroundColor: theme.colorScheme.primary,
+                  foregroundColor: theme.colorScheme.onPrimary,
+                  shape: RoundedRectangleBorder(
+                    borderRadius: BorderRadius.circular(12),
+                  ),
+                ),
+                child: const Text(
+                  'C\'est parti !',
+                  style: TextStyle(fontSize: 16, fontWeight: FontWeight.bold),
+                ),
+              ),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}
+
+class WhatsNewItem {
+  final IconData icon;
+  final String title;
+  final String description;
+
+  const WhatsNewItem({
+    required this.icon,
+    required this.title,
+    required this.description,
+  });
+}