feat: add next-intl i18n support with Polish and English

- Install next-intl and configure plugin in next.config.ts
- Move (frontend) route group into [locale] segment with as-needed prefix
- Add NextIntlClientProvider to layout with dynamic lang attribute
- Extract all hardcoded Polish UI strings into messages/pl.json
- Add English translation in messages/en.json
- Add middleware for locale detection and routing

Author: kriss145

messages/en.json

@@ -0,0 +1,41 @@
+{
+  "home": {
+    "greeting": "Hey, {name}",
+    "yourTrainingPlans": "Your training plans",
+    "noPlans": "You don't have a plan assigned yet. Contact your coach.",
+    "seriesPrefix": "Sets",
+    "repsPrefix": "Reps",
+    "durationPrefix": "Time",
+    "restPrefix": "Rest"
+  },
+  "exercise": {
+    "video": "▶ video",
+    "addSet": "+ add set",
+    "duplicateSet": "Duplicate last set"
+  },
+  "seriesForm": {
+    "note": "Note",
+    "notePlaceholder": "note",
+    "save": "Save",
+    "cancel": "Cancel",
+    "bodyweightActive": "bodyweight ✕",
+    "bodyweightTitle": "Bodyweight",
+    "atLeastOneValue": "Enter at least one value"
+  },
+  "login": {
+    "title": "Sign in",
+    "subtitle": "Enter the credentials provided by your coach.",
+    "emailLabel": "E-mail",
+    "passwordLabel": "Password",
+    "loggingIn": "Signing in…",
+    "button": "Sign in",
+    "invalidCredentials": "Invalid e-mail or password.",
+    "genericError": "Something went wrong. Please try again."
+  },
+  "nav": {
+    "logout": "Sign out"
+  },
+  "workout": {
+    "activeContext": "Active context"
+  }
+}

messages/pl.json

@@ -0,0 +1,41 @@
+{
+  "home": {
+    "greeting": "Cześć, {name}",
+    "yourTrainingPlans": "Twoje plany treningowe",
+    "noPlans": "Nie masz jeszcze przypisanego planu. Skontaktuj się z trenerem.",
+    "seriesPrefix": "Serie",
+    "repsPrefix": "Powt.",
+    "durationPrefix": "Czas",
+    "restPrefix": "Przerwa"
+  },
+  "exercise": {
+    "video": "▶ wideo",
+    "addSet": "+ dodaj serię",
+    "duplicateSet": "Duplikuj ostatnią serię"
+  },
+  "seriesForm": {
+    "note": "Notatka",
+    "notePlaceholder": "notatka",
+    "save": "Zapisz",
+    "cancel": "Anuluj",
+    "bodyweightActive": "masa własna ✕",
+    "bodyweightTitle": "Masa własnego ciała",
+    "atLeastOneValue": "Podaj co najmniej jedną wartość"
+  },
+  "login": {
+    "title": "Zaloguj się",
+    "subtitle": "Wpisz dane otrzymane od trenera.",
+    "emailLabel": "E-mail",
+    "passwordLabel": "Hasło",
+    "loggingIn": "Logowanie…",
+    "button": "Zaloguj",
+    "invalidCredentials": "Nieprawidłowy e-mail lub hasło.",
+    "genericError": "Coś poszło nie tak. Spróbuj ponownie."
+  },
+  "nav": {
+    "logout": "Wyloguj"
+  },
+  "workout": {
+    "activeContext": "Aktywny kontekst"
+  }
+}

next.config.ts

@@ -1,8 +1,11 @@
 import { withPayload } from '@payloadcms/next/withPayload'
 import type { NextConfig } from 'next'
+import createNextIntlPlugin from 'next-intl/plugin'
 import path from 'path'
 import { fileURLToPath } from 'url'
 
+const withNextIntl = createNextIntlPlugin('./src/i18n/request.ts')
+
 const __filename = fileURLToPath(import.meta.url)
 const dirname = path.dirname(__filename)
 
@@ -30,4 +33,4 @@ const nextConfig: NextConfig = {
   allowedDevOrigins: ['192.168.10.50']
 }
 
-export default withPayload(nextConfig, { devBundleServerPackages: false })
+export default withPayload(withNextIntl(nextConfig), { devBundleServerPackages: false })