feat: add user locale preference to user_preferences table (#555)

* feat: add user locale preference to user_preferences table

- Introduced a new column 'locale' in the user_preferences table to store per-user UI language preferences.
- Added a CHECK constraint to ensure only supported locales ('sv', 'en') are allowed.
- Triggered a schema reload notification for the changes.

chore: declare CSS module support in TypeScript

- Added a declaration for CSS modules in globals.d.ts to enable TypeScript support for importing CSS files.

* feat: add Swish as an invoice payment method in company settings

app/api/user/locale/route.ts

@@ -0,0 +1,44 @@
+import { NextResponse } from 'next/server'
+import { z } from 'zod'
+import { requireAuth } from '@/lib/auth/require-auth'
+import { LOCALE_COOKIE, SUPPORTED_LOCALES, type Locale } from '@/i18n/config'
+
+const BodySchema = z.object({
+  locale: z.enum(SUPPORTED_LOCALES),
+})
+
+export async function POST(request: Request) {
+  const { user, supabase, error } = await requireAuth()
+  if (error) return error
+
+  let body: unknown
+  try {
+    body = await request.json()
+  } catch {
+    return NextResponse.json({ error: 'Invalid JSON body' }, { status: 400 })
+  }
+
+  const parsed = BodySchema.safeParse(body)
+  if (!parsed.success) {
+    return NextResponse.json({ error: 'Invalid locale' }, { status: 400 })
+  }
+
+  const locale: Locale = parsed.data.locale
+
+  const { error: upsertError } = await supabase
+    .from('user_preferences')
+    .upsert({ user_id: user.id, locale }, { onConflict: 'user_id' })
+
+  if (upsertError) {
+    return NextResponse.json({ error: 'Could not save language preference' }, { status: 500 })
+  }
+
+  const response = NextResponse.json({ data: { locale } })
+  response.cookies.set(LOCALE_COOKIE, locale, {
+    path: '/',
+    sameSite: 'lax',
+    secure: process.env.NODE_ENV === 'production',
+    maxAge: 60 * 60 * 24 * 365,
+  })
+  return response
+}