feat(06-02): replace BudgetSetup with template-based month picker

- Rewrite BudgetSetup to use month picker + currency + Generate button
- Remove manual form fields (name, dates, carryover, copy-from select)
- Handle 409 conflict gracefully by calling onCreated() to refresh list
- Remove copyFrom method from budgets API (TMPL-06)
- Update BudgetSetup test to reflect new month-picker UI
- Remove copyFrom from DashboardPage test mock
- Add budget.generate, budget.month, budget.generating i18n keys (EN/DE)
- Remove budget.copyFrom and budget.setup i18n keys

frontend/src/components/BudgetSetup.test.tsx

@@ -8,8 +8,15 @@ vi.mock('react-i18next', () => ({
 
 vi.mock('@/lib/api', () => ({
   budgets: {
-    create: vi.fn().mockResolvedValue({ id: 'b1', name: 'Test Budget' }),
-    copyFrom: vi.fn().mockResolvedValue({}),
+    generate: vi.fn().mockResolvedValue({ id: 'b1', name: 'April 2026' }),
+  },
+  ApiError: class ApiError extends Error {
+    status: number
+    constructor(status: number, message: string) {
+      super(message)
+      this.name = 'ApiError'
+      this.status = status
+    }
   },
 }))
 
@@ -22,16 +29,33 @@ describe('BudgetSetup', () => {
 
   it('renders without crashing', () => {
     render(<BudgetSetup {...defaultProps} />)
-    expect(screen.getByText('budget.setup')).toBeInTheDocument()
+    const elements = screen.getAllByText('budget.generate')
+    expect(elements.length).toBeGreaterThanOrEqual(1)
   })
 
-  it.skip('shows spinner in create button when saving', () => {
-    // IXTN-01: while saving is true, the create button renders a spinner (Loader2 icon)
-    // Steps: fill required fields, click create, assert spinner is visible before resolve
+  it('shows month input and currency input', () => {
+    render(<BudgetSetup {...defaultProps} />)
+    const monthInput = document.querySelector('input[type="month"]')
+    const currencyInput = document.querySelector('input:not([type="month"])')
+    expect(monthInput).toBeInTheDocument()
+    expect(currencyInput).toBeInTheDocument()
   })
 
-  it.skip('disables create button when saving', () => {
-    // IXTN-01: while saving is true, the create button is disabled to prevent double-submit
-    // Steps: fill required fields, click create, assert button is disabled before resolve
+  it('shows Generate button (not Create)', () => {
+    render(<BudgetSetup {...defaultProps} />)
+    // The button text uses the i18n key which the mock returns as-is
+    const buttons = screen.getAllByText('budget.generate')
+    // One in header, one in button
+    expect(buttons.length).toBeGreaterThanOrEqual(1)
+  })
+
+  it('disables Generate button when month is empty', () => {
+    render(<BudgetSetup {...defaultProps} />)
+    const button = screen.getByRole('button', { name: 'budget.generate' })
+    expect(button).toBeDisabled()
+  })
+
+  it.skip('shows spinner in Generate button when saving', () => {
+    // While saving is true, the Generate button renders a spinner
   })
 })

frontend/src/components/BudgetSetup.tsx

@@ -3,9 +3,8 @@ import { useTranslation } from 'react-i18next'
 import { Card, CardContent, CardHeader, CardTitle, CardFooter } from '@/components/ui/card'
 import { Button } from '@/components/ui/button'
 import { Input } from '@/components/ui/input'
-import { Select, SelectContent, SelectItem, SelectTrigger, SelectValue } from '@/components/ui/select'
 import { Spinner } from '@/components/ui/spinner'
-import { budgets as budgetsApi, type Budget } from '@/lib/api'
+import { budgets as budgetsApi, type Budget, ApiError } from '@/lib/api'
 
 interface Props {
   existingBudgets: Budget[]
@@ -13,30 +12,24 @@ interface Props {
   onCancel: () => void
 }
 
-export function BudgetSetup({ existingBudgets, onCreated, onCancel }: Props) {
+export function BudgetSetup({ existingBudgets: _existingBudgets, onCreated, onCancel }: Props) {
   const { t } = useTranslation()
-  const [name, setName] = useState('')
-  const [startDate, setStartDate] = useState('')
-  const [endDate, setEndDate] = useState('')
+  const [month, setMonth] = useState('')
   const [currency, setCurrency] = useState('EUR')
-  const [carryover, setCarryover] = useState('0')
-  const [copyFromId, setCopyFromId] = useState('')
   const [saving, setSaving] = useState(false)
 
-  const handleCreate = async () => {
+  const handleGenerate = async () => {
     setSaving(true)
     try {
-      const budget = await budgetsApi.create({
-        name,
-        start_date: startDate,
-        end_date: endDate,
-        currency,
-        carryover_amount: parseFloat(carryover) || 0,
-      })
-      if (copyFromId) {
-        await budgetsApi.copyFrom(budget.id, copyFromId)
-      }
+      await budgetsApi.generate({ month, currency })
       onCreated()
+    } catch (err) {
+      if (err instanceof ApiError && err.status === 409) {
+        // Budget already exists for this month — navigate to it by refreshing the list
+        onCreated()
+      } else {
+        throw err
+      }
     } finally {
       setSaving(false)
     }
@@ -45,53 +38,22 @@ export function BudgetSetup({ existingBudgets, onCreated, onCancel }: Props) {
   return (
     <Card>
       <CardHeader className="bg-gradient-to-r from-violet-50 to-purple-50">
-        <CardTitle>{t('budget.setup')}</CardTitle>
+        <CardTitle>{t('budget.generate')}</CardTitle>
       </CardHeader>
       <CardContent className="flex flex-col gap-4 pt-4">
         <div className="flex flex-col gap-2">
-          <label className="text-sm font-medium">{t('budget.name')}</label>
-          <Input value={name} onChange={(e) => setName(e.target.value)} placeholder="Oktober 2025" />
+          <label className="text-sm font-medium">{t('budget.month')}</label>
+          <Input type="month" value={month} onChange={(e) => setMonth(e.target.value)} />
         </div>
-        <div className="grid grid-cols-2 gap-4">
-          <div className="flex flex-col gap-2">
-            <label className="text-sm font-medium">{t('budget.startDate')}</label>
-            <Input type="date" value={startDate} onChange={(e) => setStartDate(e.target.value)} />
-          </div>
-          <div className="flex flex-col gap-2">
-            <label className="text-sm font-medium">{t('budget.endDate')}</label>
-            <Input type="date" value={endDate} onChange={(e) => setEndDate(e.target.value)} />
-          </div>
+        <div className="flex flex-col gap-2">
+          <label className="text-sm font-medium">{t('budget.currency')}</label>
+          <Input value={currency} onChange={(e) => setCurrency(e.target.value)} />
         </div>
-        <div className="grid grid-cols-2 gap-4">
-          <div className="flex flex-col gap-2">
-            <label className="text-sm font-medium">{t('budget.currency')}</label>
-            <Input value={currency} onChange={(e) => setCurrency(e.target.value)} />
-          </div>
-          <div className="flex flex-col gap-2">
-            <label className="text-sm font-medium">{t('budget.carryover')}</label>
-            <Input type="number" step="0.01" value={carryover} onChange={(e) => setCarryover(e.target.value)} />
-          </div>
-        </div>
-        {existingBudgets.length > 0 && (
-          <div className="flex flex-col gap-2">
-            <label className="text-sm font-medium">{t('budget.copyFrom')}</label>
-            <Select value={copyFromId} onValueChange={setCopyFromId}>
-              <SelectTrigger>
-                <SelectValue placeholder="—" />
-              </SelectTrigger>
-              <SelectContent>
-                {existingBudgets.map((b) => (
-                  <SelectItem key={b.id} value={b.id}>{b.name}</SelectItem>
-                ))}
-              </SelectContent>
-            </Select>
-          </div>
-        )}
       </CardContent>
       <CardFooter className="flex gap-2 justify-end">
         <Button variant="outline" onClick={onCancel}>{t('common.cancel')}</Button>
-        <Button onClick={handleCreate} disabled={saving || !name || !startDate || !endDate} className="min-w-[120px]">
-          {saving ? <Spinner /> : t('common.create')}
+        <Button onClick={handleGenerate} disabled={saving || !month} className="min-w-[160px]">
+          {saving ? <Spinner /> : t('budget.generate')}
         </Button>
       </CardFooter>
     </Card>