v1.7.0: fix offer created_at override, invoice PDF translations, remove has_order filter

- Add created_at to offer create/update schemas so user-selected date is stored instead of always defaulting to now()
- Translate payment method in invoice PDF based on language (Příkazem → Bank transfer, etc.)
- Add order_date and cnb_rate translation keys, replace hardcoded inline strings
- Remove duplicate has_order filter from offers backend route

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/schemas/offers.schema.ts

@@ -46,6 +46,7 @@ export const CreateQuotationSchema = z.object({
     .transform((v) => Number(v))
     .refine((v) => !Number.isNaN(v), { message: "Musí být platné číslo" })
     .nullish(),
+  created_at: z.string().nullish(),
   valid_until: z.string().nullish(),
   currency: z.string().optional().default("CZK"),
   language: z.string().optional().default("cs"),
@@ -76,6 +77,7 @@ export const UpdateQuotationSchema = z.object({
     .transform((v) => Number(v))
     .refine((v) => !Number.isNaN(v), { message: "Must be a valid number" })
     .optional(),
+  created_at: z.union([z.string(), z.null()]).optional(),
   valid_until: z.union([z.string(), z.null()]).optional(),
   currency: z.string().optional(),
   language: z.string().optional(),