feat: editable billing text on invoices

- Added billing_text column to invoices table (VARCHAR 500)
- Prisma migration: 20260323_add_billing_text
- Form field on invoice create page with placeholder
- PDF uses billing_text, falls back to default translation
- Stored on create and editable on draft invoices

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/admin/pages/InvoiceDetail.tsx

@@ -77,6 +77,7 @@ interface InvoiceForm {
   payment_method: string
   constant_symbol: string
   issued_by: string
+  billing_text: string
   notes: string
   bank_account_id: number | string
   bank_name: string
@@ -271,6 +272,7 @@ export default function InvoiceDetail() {
     payment_method: 'Příkazem',
     constant_symbol: '0308',
     issued_by: user?.fullName || '',
+    billing_text: '',
     notes: '',
     bank_account_id: '',
     bank_name: '',
@@ -855,6 +857,16 @@ export default function InvoiceDetail() {
                 </FormField>
               </div>
 
+              <FormField label="Text fakturace (na PDF)">
+                <input
+                  type="text"
+                  value={form.billing_text}
+                  onChange={(e) => setForm(prev => ({ ...prev, billing_text: e.target.value }))}
+                  className="admin-form-input"
+                  placeholder="Fakturujeme Vám za: (ponechte prázdné pro výchozí)"
+                />
+              </FormField>
+
               <div className="admin-form-row">
                 <FormField label="Datum vystavení" error={errors.issue_date} required>
                   <AdminDatePicker mode="date" value={form.issue_date} onChange={(val: string) => { setForm(prev => ({ ...prev, issue_date: val })); setErrors(prev => ({ ...prev, issue_date: '' })) }} />