Implement internationalization support across components

- Integrated `next-intl` for toast messages and locale handling in various components, including `Providers`, `CanvasUserMenu`, and `CreditOverview`.
- Replaced hardcoded strings with translation keys to enhance localization capabilities.
- Updated `RootLayout` to dynamically set the language attribute based on the user's locale.
- Ensured consistent user feedback through localized toast messages in actions such as sign-out, canvas operations, and billing notifications.

convex/schema.ts

@@ -310,4 +310,16 @@ export default defineSchema({
     concurrentJobs: v.number(),                    // Aktuell laufende Jobs
   })
     .index("by_user_date", ["userId", "date"]),
+
+  // ==========================================================================
+  // User Settings
+  // ==========================================================================
+
+  userSettings: defineTable({
+    userId: v.string(),                            // Better Auth User ID
+    locale: v.optional(v.union(v.literal('de'), v.literal('en'))),
+    createdAt: v.number(),
+    updatedAt: v.number(),
+  })
+    .index("by_user", ["userId"]),
 });