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/users.ts

@@ -0,0 +1,47 @@
+import { mutation } from "./_generated/server";
+import { v } from "convex/values";
+import { requireAuth } from "./helpers";
+
+export const setLocale = mutation({
+  args: {
+    locale: v.union(v.literal("de"), v.literal("en")),
+  },
+  handler: async (ctx, args) => {
+    const { userId } = await requireAuth(ctx);
+
+    const existing = await ctx.db
+      .query("userSettings")
+      .withIndex("by_user", (q) => q.eq("userId", userId))
+      .unique();
+
+    const now = Date.now();
+
+    if (existing) {
+      await ctx.db.patch(existing._id, {
+        locale: args.locale,
+        updatedAt: now,
+      });
+    } else {
+      await ctx.db.insert("userSettings", {
+        userId,
+        locale: args.locale,
+        createdAt: now,
+        updatedAt: now,
+      });
+    }
+  },
+});
+
+export const getLocale = mutation({
+  args: {},
+  handler: async (ctx) => {
+    const { userId } = await requireAuth(ctx);
+
+    const existing = await ctx.db
+      .query("userSettings")
+      .withIndex("by_user", (q) => q.eq("userId", userId))
+      .unique();
+
+    return existing?.locale ?? null;
+  },
+});