Pluralise counts properly instead of writing dag(ar)

The translate function learns a {count|singular|plural} form, and the
24 strings that hedged with "dag(ar)", "day(s)" or "week(s)" now pick
the right word from the number they interpolate, in both languages.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XBRLomdLK9jF7wU7a8AA6H

frontend/src/pages/Calendar.test.tsx

@@ -1166,7 +1166,7 @@ describe('Calendar', () => {
 
       const toast = await screen.findByTestId('calendar-toast')
       expect(toast).toHaveTextContent('7')
-      expect(toast).toHaveTextContent('dagarna')
+      expect(toast).toHaveTextContent('dagar')
       expect(toast).not.toHaveTextContent('3 vecko')
       expect(toast).not.toHaveTextContent('veckorna')
     })
@@ -1218,7 +1218,7 @@ describe('Calendar', () => {
       fireEvent.click(screen.getByTestId('preset-click'))
 
       const toast = await screen.findByTestId('calendar-toast')
-      // What changed (existing "N day(s) changed" summary) ...
+      // What changed (the "N days changed" summary with pluralization) ...
       expect(toast).toHaveTextContent('ändrades')
       // ... and what fell short.
       expect(toast).toHaveTextContent('5')