feat(smart-app): implement complete mobile app MVP

- App.tsx: full navigation (Auth stack + Main tabs with 5 screens)
- Auth: LoginScreen, RegisterScreen, ForgotPasswordScreen
- HomeScreen: dashboard with IoT metrics, weather widget, alerts, quick actions, sensors
- MapScreen: interactive map with layer toggles (6 layers)
- MarketplaceScreen: categories (6), products (5), search
- ChatScreen: AI chat with quick prompts (4), bot responses
- ProfileScreen: user info, stats, menu (9 items), logout
- AlertsScreen: alert list with severity, acknowledge
- SensorsScreen: sensor list with type filters (6 types), search
- ZonesScreen: zone cards with stats
- SettingsScreen: language picker (FR/EN/ES/DE), privacy, about
- Stores: iotStore (sensors, zones, alerts), notificationStore, uiStore + i18n
- Hooks: useSensors, useAlerts, useNotifications, useLocation
- Components: Card, Button, LoadingSpinner, ErrorBoundary, Header
- Services: iotService, notificationService (with axios API client)
- Utils: formatters (temp, AQI, noise, dates), validators (email, password, IBAN)
- Theme: colors.ts with full design system (Blue Ocean palette)
- Ditto: fixed MongoDB connection, new JWT secrets, official gateway image

smart-app-city/frontend/src/utils/constants.ts

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+// TODO: Implement

smart-app-city/frontend/src/utils/formatters.ts

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+// Smart App City — Formatters utility
+
+export const formatters = {
+  temperature: (value: number, unit: string = '°C') => `${value.toFixed(1)}${unit}`,
+  humidity: (value: number) => `${value}%`,
+  aqi: (value: number) => {
+    if (value <= 50) return { label: 'Bon', color: '#2E7D32' };
+    if (value <= 100) return { label: 'Modéré', color: '#F57C00' };
+    if (value <= 150) return { label: 'Mauvais', color: '#D32F2F' };
+    return { label: 'Dangereux', color: '#7B1FA2' };
+  },
+  noise: (value: number) => {
+    if (value <= 40) return { label: 'Silencieux', color: '#2E7D32' };
+    if (value <= 55) return { label: 'Normal', color: '#0288D1' };
+    if (value <= 70) return { label: 'Bruyant', color: '#F57C00' };
+    return { label: 'Très bruyant', color: '#D32F2F' };
+  },
+  date: (date: string | Date, locale: string = 'fr-FR') => {
+    const d = new Date(date);
+    return d.toLocaleDateString(locale, {
+      day: '2-digit',
+      month: '2-digit',
+      year: 'numeric',
+      hour: '2-digit',
+      minute: '2-digit',
+    });
+  },
+  time: (date: string | Date) => {
+    const d = new Date(date);
+    return d.toLocaleTimeString('fr-FR', { hour: '2-digit', minute: '2-digit' });
+  },
+  relativeTime: (date: string | Date) => {
+    const d = new Date(date);
+    const now = new Date();
+    const diffMs = now.getTime() - d.getTime();
+    const diffMins = Math.floor(diffMs / 60000);
+    const diffHours = Math.floor(diffMs / 3600000);
+    const diffDays = Math.floor(diffMs / 86400000);
+
+    if (diffMins < 1) return "À l'instant";
+    if (diffMins < 60) return `Il y a ${diffMins} min`;
+    if (diffHours < 24) return `Il y a ${diffHours}h`;
+    if (diffDays < 7) return `Il y a ${diffDays}j`;
+    return d.toLocaleDateString('fr-FR', { day: '2-digit', month: '2-digit' });
+  },
+  currency: (value: number, currency: string = 'EUR') =>
+    new Intl.NumberFormat('fr-FR', { style: 'currency', currency }).format(value),
+};

smart-app-city/frontend/src/utils/validators.ts

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+// Smart App City — Validators utility
+
+export const validators = {
+  email: (email: string): boolean => {
+    const re = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
+    return re.test(email);
+  },
+  password: (password: string): { valid: boolean; errors: string[] } => {
+    const errors: string[] = [];
+    if (password.length < 8) errors.push('Au moins 8 caractères');
+    if (!/[A-Z]/.test(password)) errors.push('Au moins une majuscule');
+    if (!/[a-z]/.test(password)) errors.push('Au moins une minuscule');
+    if (!/[0-9]/.test(password)) errors.push('Au moins un chiffre');
+    return { valid: errors.length === 0, errors };
+  },
+  name: (name: string): boolean => {
+    return name.trim().length >= 2;
+  },
+  required: (value: string): boolean => {
+    return value.trim().length > 0;
+  },
+  iban: (iban: string): boolean => {
+    const cleaned = iban.replace(/\s/g, '').toUpperCase();
+    if (!/^[A-Z]{2}[0-9]{2}[A-Z0-9]{4}[0-9]{7}([A-Z0-9]?){0,16}$/.test(cleaned)) return false;
+    // IBAN checksum validation
+    const rearranged = cleaned.slice(4) + cleaned.slice(0, 4);
+    const numeric = rearranged.replace(/[A-Z]/g, (c) => (c.charCodeAt(0) - 55).toString());
+    let remainder = numeric;
+    while (remainder.length > 2) {
+      const block = remainder.slice(0, 9);
+      remainder = (parseInt(block, 10) % 97).toString() + remainder.slice(block.length);
+    }
+    return parseInt(remainder, 10) % 97 === 1;
+  },
+};