Translate UI and user-facing messages to Ukrainian (#5)

- All frontend pages, labels, notices and errors; html lang=uk, uk-UA money format
- Brand "Assistant System" in the top bar and page title
- Backend error details returned to the UI (auth, orders, receipts,
  cash registers, Checkbox/CRM/NP errors) and CLI output
- Tests updated for the new messages

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/schemas/auth.py

@@ -22,7 +22,7 @@ class TokenPair(BaseModel):
     access_token: str
     refresh_token: str
     token_type: str = "bearer"
-    expires_in: int = Field(description="Время жизни access-токена в секундах")
+    expires_in: int = Field(description="Час життя access-токена в секундах")
 
 
 class UserOut(BaseModel):