Fix: surface the real booking error instead of generic 'Boeken mislukt'

src/ui/hooks/useBooking.test.ts

@@ -190,8 +190,17 @@ describe('useBooking', () => {
     expect(result.current.errorMessage).toContain('API key')
   })
 
-  it('book sets a default error message for non-Error throws', async () => {
-    bookExecute.mockRejectedValue('weird')
+  it('book surfaces a string error (e.g. the Simplicate API message) instead of a generic one', async () => {
+    bookExecute.mockRejectedValue('Simplicate API error: 400 — invalid type_id')
+    const { result } = renderHook(() => useBooking({ projectId: 'p1' }))
+    await act(async () => {
+      await result.current.book()
+    })
+    expect(result.current.errorMessage).toBe('Simplicate API error: 400 — invalid type_id')
+  })
+
+  it('book falls back to a generic message for non-string, non-Error throws', async () => {
+    bookExecute.mockRejectedValue({ weird: true })
     const { result } = renderHook(() => useBooking({ projectId: 'p1' }))
     await act(async () => {
       await result.current.book()

src/ui/hooks/useBooking.ts

@@ -130,7 +130,12 @@ export function useBooking(initial: Partial<HourEntry> = {}) {
       setStatus('success')
     } catch (err) {
       setStatus('error')
-      setErrorMessage(err instanceof Error ? err.message : 'Boeken mislukt')
+      // Tauri's invoke rejects with the Rust error STRING (e.g. the Simplicate
+      // API status + body), not an Error — surface it instead of hiding the real
+      // reason behind a generic message.
+      console.error('[useBooking] booking failed:', err)
+      const msg = err instanceof Error ? err.message : typeof err === 'string' && err.trim() ? err : 'Boeken mislukt'
+      setErrorMessage(msg)
     }
   }